GoogleTest: Avoid silent failures of dynamic test discovery

The internal helper variable '_GOOGLETEST_DISCOVER_TESTS_SCRIPT' can have gone
out-of-scope when 'gtest_discover_tests()' is called, depending on where the
GoogleTest module is actually included. This leads to a silent failure of
dynamic test discovery, since the custom post-build commands actually does
nothing (it basically invokes 'cmake -P ""'). Ctest will then fail to run the
tests, considering them to be 'not built'.

Fix this by determining the path to the GoogleTest module based on
'${CMAKE_ROOT}' instead, which is always available.

A new test case was added to test suite 'RunCMake/GoogleTest' to ensure that
'gtest_discover_tests()' works correctly when invoked in a different variable
scope.

Fixes: #25477

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