Untainted file passed to unlink in t/config-finder.t re gh #438

Hopefully fixes #438 - but **please review for security**. `-f` may not be sufficient as a taint check (although perl accepts it as such). It does prevent fs destruction described at the documentation for `unlink`, and I think the fact that it is (I think) removing files previously created by the test file is probably ok, but I am not sure.