added a test for teuthology.orchestra.cluster.write_file

Signed-off-by: Andrew Schoen <aschoen@redhat.com>
This probably should be another test as you are mixing cases and assertions. For example a test named `test_fails_with_invalid_perms` that has this case in would make it obvious when that fails (as to what was it that caused the failure) vs. having one test that has multiple cases/assertions in it.