Problem:
RunTests.cmake adds $TEST_PATH to $TMPDIR with the implication that it
gives more isolation. But this is misleading because $TEST_PATH is only
defined once. Full test runs use the same $TMPDIR for all tests.
This was likely added with the intention of invoking RunTests.cmake
once-per-testfile from a wrapper than does the isolation/orchestration.
But even so, Nvim's vim_maketempdir() / vim_mktempdir() _already_
creates a unique tempdir per session.
Solution:
Don't append $TEST_PATH to $TMPDIR. Avoids confusion and makes the path
shorter.