Ensure server tests and fixtures properly clean up the global flask app

The global variable vault gets reused across test runs, so it needs to be reset on fixture/test teardown, or any potential new config may not be taken into account, which can in turn generate confusing postgresql connection closures.

Depends on !141 (closed)

Test Plan

pytest with pytest-random-order

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Migrated from D7292 (view on Phabricator)