MySQL: Figure out how to handle tests that modify schema

[killercup]

Our test suite for Postgres and SQLite assumes we can modify the db schema inside a transaction. This does not work on MySQL (it commits the transaction when you try to modify the schema).

We need to figure out a way to have those tests run on MySQL without introducing a bunch of overhead.

(To find affected tests, grep for FIXME: Figure out how to handle tests that modify schema)