Improve examples

* Simpler and standard build workflow
* Utilize CTest with the recommended workflow of optional tests
* Skip using make directly so these become more platform portable
* Skip having to manually create the build directory
* Do all operations from the source directory when possible to reduce
  the number of commands

Signed-off-by: Ryan Friedman <ryanfriedman5410+github@gmail.com>

examples/standalone/acoustic_comms_demo/README.md

@@ -15,17 +15,19 @@ will receive the signal and start moving before Daphne.
 
 From this directory, run the following to compile:
 
-    mkdir build
-    cd build
-    cmake ..
-    make
+```bash
+cmake -B build
+cmake --build build
+```
 
 ## Execute Instructions
 
-From the `build` directory, run Gazebo Sim and the example controller:
+Run Gazebo Sim and the example controller:
 
-    gz sim -r ../../../worlds/acoustic_comms_demo.sdf
-    ./acoustic_comms_demo
+```bash
+gz sim -r ../../worlds/acoustic_comms_demo.sdf
+./build/acoustic_comms_demo
+```
 
 It can be seen visually that one of the vehicles (Triton) starts moving
 immediately, then afer a while Tethys will start moving, and then finally Daphne.

examples/standalone/custom_server/README.md

@@ -5,17 +5,18 @@ using the C++ API, instead of using Gazebo from the command line.
 
 ## Build Instructions
 
-From this directory:
+From this directory, run the following to compile:
 
-    cd gz-sim/examples/standalone/custom_server
-    mkdir build
-    cd build
-    cmake ..
-    make
+```bash
+cmake -B build
+cmake --build build
+```
 
 ## Execute Instructions
 
-    ./custom_server
+```bash
+./build/custom_server
+```
 
 The server will run `shapes.sdf` for 100 iterations and exit. No GUI will
 show up.

examples/standalone/each_performance/README.md

@@ -1,18 +1,33 @@
 Example program to generate performance data
 
-### Build
+## Build Instructions
 
-    cd examples/standalong/each_performance
-    mkdir build
-    cd build
-    cmake ..
-    make
+From this directory, run the following to compile:
+
+```bash
+cmake -B build
+cmake --build build
+```
 
 ### Run
 
-    ./each
+```bash
+cd build
+./each
+```
+
+This will generate `./build/each.data` which is the performance data.
+
+### Generate and view plots
+
+One the above program is complete, generate the plots.
+
+```bash
+cd build
+# Generate plots
+gnuplot -e "filename='each.data'" ../each.gp
+eog *.png
+```
 
-### Generate and view plot
+You can now scroll through two plots to view performance data.
 
-    gnuplot -e "filename='each.data'" ../each.gp
-    eog *.png

examples/standalone/entity_creation/README.md

@@ -2,24 +2,21 @@
 
 ## Build
 
-```
-mkdir build
-cd build
-cmake ..
-make
+```bash
+cmake -B build
+cmake --build build
 ```
 
 ## Run
 
 This example only works if the world is called `empty`. Start an empty world with:
 
-```
+```bash
 gz sim empty.sdf
 ```
 
 Then run the create program to spawn entities into the world:
 
-```
-cd build
-./entity_creation
+```bash
+./build/entity_creation
 ```

examples/standalone/external_ecm/README.md

@@ -3,35 +3,35 @@
 Example showing how to get a snapshot of all entities and components in a
 running simulation from an external program using the state message.
 
-## Build
+## Build Instructions
 
-From the root of the `gz-sim` repository, do the following to build the example:
+From this directory, run the following to compile:
 
-~~~
-cd gz-sim/examples/standalone/external_ecm
-mkdir build
-cd build
-cmake ..
-make
-~~~
+```bash
+cmake -B build
+cmake --build build
+```
 
 This will generate the `external_ecm` executable under `build`.
 
 ## Run
 
 Start a simulation, for example:
 
-    gz sim shapes.sdf
+```bash
+gz sim shapes.sdf
+```
 
-On another terminal, run the `external_ecm` executable, passing the name of the
+In another terminal, from this directory, run the `external_ecm` executable, passing the name of the
 running world you want to inspect:
 
-  cd gz-sim/examples/standalone/external_ecm
-  ./external_ecm shapes
+```bash
+./build/external_ecm shapes
+```
 
 You should see something like this:
 
-```
+```text
 $ ./external_ecm shapes
 
 Requesting state for world [shapes] on service [/world/shapes/state]...

examples/standalone/gtest_setup/CMakeLists.txt

@@ -1,37 +1,33 @@
-cmake_minimum_required(VERSION 3.11.0 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.16 FATAL_ERROR)
 
 project(GTestSetup)
 
 # Find Gazebo
 find_package(gz-sim8 REQUIRED)
 set(GZ_SIM_VER ${gz-sim8_VERSION_MAJOR})
 
-# Fetch and configure GTest
-include(FetchContent)
-FetchContent_Declare(
-  googletest
-  # Version 1.14. Use commit hash to prevent tag relocation
-  URL https://github.com/google/googletest/archive/f8d7d77c06936315286eb55f8de22cd23c188571.zip
-)
-set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
-FetchContent_MakeAvailable(googletest)
+find_package(GTest REQUIRED)
+if(NOT TARGET GTest::gtest_main AND TARGET gtest_main)
+  add_library(GTest::gtest_main ALIAS gtest_main)
+endif()
 
-enable_testing()
-include(Dart)
+include(CTest)
+# CTest defines "BUILD_TESTING" so that users can easily disable tests in a common way.
+if(BUILD_TESTING)
+  # Generate tests
+  foreach(TEST_TARGET
+    command_TEST
+    gravity_TEST)
 
-# Generate tests
-foreach(TEST_TARGET
-  command_TEST
-  gravity_TEST)
-
-  add_executable(
-    ${TEST_TARGET}
-    ${TEST_TARGET}.cc
-  )
-  target_link_libraries(${TEST_TARGET}
-    gtest_main
-    gz-sim${GZ_SIM_VER}::gz-sim${GZ_SIM_VER}
-  )
-  include(GoogleTest)
-  gtest_discover_tests(${TEST_TARGET})
-endforeach()
+    add_executable(
+      ${TEST_TARGET}
+      ${TEST_TARGET}.cc
+    )
+    target_link_libraries(${TEST_TARGET}
+      GTest::gtest_main
+      gz-sim${GZ_SIM_VER}::gz-sim${GZ_SIM_VER}
+    )
+    include(GoogleTest)
+    gtest_discover_tests(${TEST_TARGET})
+  endforeach()
+endif()

examples/standalone/gtest_setup/README.md

@@ -9,18 +9,37 @@ The example contains 2 tests:
 
 See the comments on the source code for more explanations.
 
-## Build
+## Build Instructions
 
-From the root of the repository:
+From this directory, run the following to compile:
 
-    cd examples/standalone/gtest_setup
-    mkdir build
-    cd build
-    cmake ..
-    make
+```bash
+cmake -B build
+cmake --build build
+```
 
 ## Run tests
 
-    cd examples/standalone/gtest_setup/build
-    ./gravity_TEST
-    ./command_TEST
+From this directory, run the following to run tests:
+
+### CMake 3.20 or newer
+
+```bash
+ctest --test-dir build
+```
+
+By default, ctest hides stdout from tests.
+To enable test output, add `-V`.
+CTest also hides colors, which can be re-enabled.
+```bash
+GTEST_COLOR=1 ctest -V --test-dir build 
+```
+
+
+### CMake 3.19 or earlier
+
+```bash
+cd build
+ctest
+```
+

examples/standalone/joy_to_twist/README.md

@@ -6,17 +6,14 @@ messages and converts publishes
 [gz::msgs::Twist](https://gazebosim.org/api/msgs/9/classgz_1_1msgs_1_1Twist.html)
 messages according to user-defined configuration.
 
-## Build
+## Build Instructions
 
-From the root of the `gz-sim` repository, do the following to build the example:
+From this directory, run the following to compile:
 
-~~~
-cd examples/standalone/joy_to_twist
-mkdir build
-cd build
-cmake ..
-make
-~~~
+```bash
+cmake -B build
+cmake --build build
+```
 
 This will generate the `joy_to_twist` executable under `build`.
 
@@ -27,7 +24,9 @@ An example file, `joy_to_twist.sdf`, is provided.
 
 You can run the example as follows:
 
-    ./joy_to_twist ../joy_to_twist.sdf
+```bash
+./build/joy_to_twist joy_to_twist.sdf
+```
 
 This program by itself won't do much unless there is another source of joy
 messages that it can consume. See the demo below for a full integrated example.
@@ -41,8 +40,10 @@ that can be controlled using a joystick. You can run it as follows:
    messages. See that standalone program's instructions to details on how
    to build it. Once it's built, you can run it as follows:
 
-        cd examples/standalone/joystick
-        ./joystick ../joystick.sdf
+```bash
+cd examples/standalone/joystick
+./build/joystick ../joystick.sdf
+```
 
 1. On another terminal, run the `joy_to_twist` executable as described above,
    which will convert joy messages to twist messages: