Merge #18829: ci: Merge C++17 build with one of the existing ones

aaaacff ci: Merge C++17 build with one of the existing ones (MarcoFalke)

Pull request description:

  No need to spin up an extra vm for each pull request for a simple sanity check that any of the other already running machines can test.

Top commit has no ACKs.

Tree-SHA512: ce1609aa4cbc057fc9e85e61f300eac2317cc206647a20f6b001180ffed6623e2243a408476546e16baacb91cb2dd9a2e7b6e2402c8fd829a72860b2c3eb7be6

.travis.yml

@@ -128,7 +128,7 @@ jobs:
         FILE_ENV="./ci/test/00_setup_env_i686_centos.sh"
 
     - stage: test
-      name: 'x86_64 Linux  [GOAL: install]  [bionic]  [previous releases, uses qt5 dev package and some depends packages] [unsigned char]'
+      name: 'x86_64 Linux  [GOAL: install]  [bionic]  [C++17, previous releases, uses qt5 dev package and some depends packages] [unsigned char]'
       env: >-
         FILE_ENV="./ci/test/00_setup_env_native_qt5.sh"
 
@@ -148,11 +148,6 @@ jobs:
       env: >-
         FILE_ENV="./ci/test/00_setup_env_native_valgrind.sh"
 
-    - stage: test
-      name: 'x86_64 Linux  [GOAL: install]  [bionic]  [no depends, only system libs, c++17]'
-      env: >-
-        FILE_ENV="./ci/test/00_setup_env_native_cxx17.sh"
-
     - stage: test
       name: 'x86_64 Linux  [GOAL: install]  [focal]  [no depends, only system libs, sanitizers: fuzzer,address,undefined]'
       env: >-

ci/test/00_setup_env_native_qt5.sh

@@ -14,4 +14,4 @@ export RUN_UNIT_TESTS_SEQUENTIAL="true"
 export RUN_UNIT_TESTS="false"
 export GOAL="install"
 export TEST_PREVIOUS_RELEASES=true
-export BITCOIN_CONFIG="--enable-zmq --with-gui=qt5 --enable-glibc-back-compat --enable-reduce-exports --enable-debug CFLAGS=\"-g0 -O2 -funsigned-char\" CXXFLAGS=\"-g0 -O2 -funsigned-char\""
+export BITCOIN_CONFIG="--enable-zmq --with-gui=qt5 --enable-glibc-back-compat --enable-reduce-exports --enable-c++17 --enable-debug CFLAGS=\"-g0 -O2 -funsigned-char\" CXXFLAGS=\"-g0 -O2 -funsigned-char\""