[unittests] Use GTEST_SKIP() instead of return when appropriate

Basically NFC: A TEST/TEST_F/etc that bails out early (usually because
setup failed or some other runtime condition wasn't met) generally
should use GTEST_SKIP() to report its status correctly, unless it
takes steps to report another status (e.g., FAIL()).

I did see a handful of tests show up as SKIPPED after this change,
which is not unexpected. The status seemed appropriate in all the new
cases.