ctest_coverage_collect_gcov's GCOV_OPTIONS should default to CTEST_COVERAGE_EXTRA_FLAGS
https://cmake.org/cmake/help/latest/manual/ctest.1.html#ctest-coverage-step defaults to CTEST_COVERAGE_COMMAND and CTEST_COVERAGE_EXTRA_FLAGS
https://cmake.org/cmake/help/latest/module/CTestCoverageCollectGCOV.html defaults to CTEST_COVERAGE_COMMAND, but GCOV_OPTIONS
doesn't default to CTEST_COVERAGE_EXTRA_FLAGS, which feels strange.