File-Find/t - rework tempdir creation and cleanup

File-Find/t - rework tempdir creation and cleanup

Fixes #20734 - in a previous patch I missed the early cleanup call
and the fact that it could result in a race condition. This hopefully
resolves the problem.

These tests files are pretty crufty. It would be nice to see them split
apart so that the "sanity" checks which expect to be run in t/ are
executed in a separate test files from the checks which build a tree to
traverse for testing. A perfect task for a new contributor.