Remove users from dumps of opt-out (bug 1094971)

I'm not pleased to get a "so whatever" on a PR and thinking that I'm doing something wrong. Is there another way you suggest this be written? Adding the `tearDown` code seemed cleaner to me as it cleans up after itself -- the top level `_polite_tmpdir` is still there but the files within it are now cleaned up after each test. I'm open to suggestions if this isn't ideal.