GoogleTest: Avoid POST_BUILD race condition for gtest_discover_tests()

If multiple calls to `gtest_discover_tests()` are made with different
targets and they use the same working directory, they would previously
have tried to use the same `cmake_test_discovery.json` file for collecting
the set of tests during discovery. Incorporate a hash of the target name
into the file name to ensure that no longer occurs.

Fixes: #27319