test: fix temp-dir option in tools/test.py

If a temp-dir is specified and already exists, the NODE_TEST_DIR
environment variable will never be set. This fixes 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>