Revert "Cleanup: move CMake test utility functions into testing.cmake"

This breaks execution of some Windows tests.

This reverts commit 4190a61020.

intern/atomic/CMakeLists.txt

@@ -41,5 +41,6 @@ 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()