Extend test DB cleanup to drop all tables.
Extend test DB cleanup to drop all tables.

I ran into this problem when removing the Measure model. Since the model
wasn't there anymore `_Model.metadata.drop_all(engine)` didn't remove it.
This left the measure table orphaned in the test database. Now we actually
remove all tables in the test database, not just those known to our current
model metadata.

mozilla/ichnaea,mozilla/ichnaea,mozilla/ichnaea,mozilla/ichnaea,therewillbecode/ichnaea,therewillbecode/ichnaea,therewillbecode/ichnaea