gtest_discover_tests: Re-run PRE_TEST discovery on any arg change
Given the increased likelihood of users hitting #22912 (closed) with the new TEST_FILTER
feature added by !6406 (merged) for the next release, we probably should include this fix in CMake 3.22.0.
Also included is a drive-by fix to one of the GoogleTest test cases, which had a misnamed file that should have been verifying the expected output but wasn't.
Backport: release