FindGTest: Allow either "Debug" or "Release" configurations.
Call select_library_configurations on GTest library variable basenames "GTEST" and "GTEST_MAIN" before passing ${basename}_LIBRARY variables to find_package_handle_standard_args. This allows either _DEBUG or _RELEASE library variants to be found for find_package() to pass, whereas the original version would require the presence of _RELEASE variants.
Fixes: #17799