test: use tmpDir in test-fs-utimes

test-fs-utimes was doing some tests against __filename. This made the
test unreliable when multiple copies were run simultaneously. In
general, tests should use files in either the tmp directory or else
fixtures, so change to using `common.tmpDir` instead. Each copy of the
test (if using `test.py` harness for parallel runs) will use its own
directory, making the test robust again.

PR-URL: https://github.com/nodejs/node/pull/16774


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>