tests: Remove pam-wrapper temporary folder on test completion
PAM wrapper creates /tmp/pam.X files during its execution (strictly as it does not follow $TMPDIR either), however given the low number of combinations, we may end up in re-using the same pam.* folder during meson test, causing a failure.
As per this, remove these temporary files on tearDown so that we won't try reusing the same folder multiple times.