Use enable_testing() instead of include(CTest)

[jpsamper2009]

• include(CTest) enables CTest and CDash
• CDash creates a lot of targets (ContinuousBuild, NightlyTest, etc.) which are not needed
• enable_testing() enables CTest without CDash

See also https://stackoverflow.com/questions/45169854/cmake-in-qtcreator-4-3-shows-many-automatic-targets-how-to-remove-hide-them

[jpsamper2009]

Turns out that this change disables all tests that are wrapped with BUILD_TESTING. The alternative may be to set(ENABLE_TESTING ON) also, but I'll investigate further to make sure we don't have even more unexpected side effects

[dirk-thomas]

Instead of calling set(...) you should define it with option(...) as it happens when including the CTest module.

[jpsamper2009]

@dirk-thomas It looks like there is some expectation that CDash is enabled. I get the following warning after adding option(BUILD_TESTING "Option to enable/disable tests" ON):

$ colcon test --packages-select rcutils
...
--- stderr: rcutils
Cannot find file: /home/jp.samper/ros2_ws/build/rcutils/DartConfiguration.tcl
Problem reading custom configuration: /home/jp.samper/ros2_ws/build/rcutils/CTestCustom.ctest
Cannot find file: /home/jp.samper/ros2_ws/build/rcutils/DartConfiguration.tcl
---

• The CTestCustom.ctest warning is coming from line 27 of ament_cmake_test-extras.cmake
  • Simply deleting the lines gets rid of the warning without causing any new warnings/error
• The other two warnings are generated by the ctest invocation because of the -D ExperimentalTest.
  • Removing -D ExperimentalTest from colcon_cmake (https://github.com/colcon/colcon-cmake/blob/master/colcon_cmake/task/cmake/test.py#L71) would get rid of the warning

Since it looks like this is a bigger task than expected. I'll actually close this PR and create an issue for discussing.