t3600: clean up permissions test properly

t3600: clean up permissions test properly

The test of failing `git rm -f` removes the write permissions on the
test directory, but fails to restore them if the test fails. This
means that the test temporary directory cannot be cleaned up, which
means that subsequent attempts to run the test fail mysteriously.

Instead, do the cleanup in a `test_when_finished` block so that it
can't be skipped.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>