Fix migration tests blocking infinitely if migrations fail

Created by: mrnugget

I ran into this this morning (see Slack): migration was bad but I couldn't find out why my tests were failing (locally and on Ci). No error message was printed, just a timeout after 10min.

Walking up and down the stack trace I ended up here: if the up/down migration would fail, the connection/transaction was aborted and the next query would block until test timeout.

This changes the tests back to the old layout, because since they both share the same global test database a failure in one would also influence/block the other test.