gtestutils: Add g_test_get_path() API

I found myself wanting to know the test that is currently being run,
where e.g. __func__ would be inconvenient to use, because e.g. the place
the string was needed was not in the test case function. Using __func__
also relies on the test function itself containing the whole path, while
loosing the "/" information that is part of the test path.
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