R1:37f1f555577c

Fix a possible deadlock in unit tests after an error

Summary:
After certain types of errors, we may deadlock when trying to destroy test databases.

Specifically, we still have connections open to, say, `phabricator_unittest_abasonaknlbaklnasb_herald` (or whatever) and MySQL sometimes (not sure exactly when?) waits for them before destorying the database.

Test Plan:
- Added `$m = null; $m->method()` to a fixture test to force a fatal.
- Saw consistent deadlock, with `storage destroy` never exiting.