test: remove unused testpy code

The temporary directory for tests is managed in the JS `common` module.
Remove unused variable from `testpy` that is legacy from when it was
managed in the Python test harness.

PR-URL: https://github.com/nodejs/node/pull/12844


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Cai <davidcai1993@yahoo.com>