test: use realpath for NODE_TEST_DIR in common.js

If you don't specify NODE_TEST_DIR, common.tmpDir will resolve to the
realpath of `node/test`.

If you do specify NODE_TEST_DIR (with the environment variable or by
running or by running tools/test.py --test-dir=x), common.tmpDir (which
is resolved from testRoot) uses the symbolic path (doesn't resolve
symlinks). This uses fs.realpathSync() to fix that.

PR-URL: https://github.com/nodejs/node/pull/10723


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>