test: clarify usage of tmpdir.refresh()

This emphasizes that `tmpdir.refresh()` must be called only once in each
test file when needed.

PR-URL: https://github.com/nodejs/node/pull/37383


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>