Cleanup: move CMake test utility functions into testing.cmake

Combining functions from macros.cmake and Modules/GTestTesting.cmake.
It was unusual to have Blender specific code in the Modules folder.

Pull Request: https://projects.blender.org/blender/blender/pulls/116719

intern/atomic/CMakeLists.txt

@@ -41,6 +41,5 @@ if(WITH_GTESTS)
   set(TEST_LIB
     PRIVATE bf_intern_atomic
   )
-  include(GTestTesting)
   blender_add_test_executable(atomic "${TEST_SRC}" "${INC};${TEST_INC}" "${INC_SYS}" "${LIB};${TEST_LIB}")
 endif()