test: add missing tmpdir.refresh() in recently-added test

Without `tmpdir.refresh()`, the test fails in some situations. This was
missed because using `test.py` will almost always result in a leftover
tmpdir lying around that makes the `refresh()` not needed.

Refs: https://github.com/nodejs/node/pull/24913#issuecomment-447982921

PR-URL: https://github.com/nodejs/node/pull/25098


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>