Destroy fixture explicitly

Summary:
Unittest databases are not always destroyed in our setup.
It could be caused by __destruct() not called in case of a fatal error.

Test Plan:

arc unit src/applications/calendar/storage/holiday

Reviewers: edward, epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2568