Turn `try-finally` in tests back into `try-except`
Turn `try-finally` in tests back into `try-except`

It turns out that making this a `try-finally` was a misreading of the
prior logic. `finally` runs even if the `try` succeeds, of course, but
`self._clean()` should only run if things fail -- `tearDown` handles
running it otherwise.

globus/globus-cli,globus/globus-cli