Tests need to be faster

[JasperHorn]

Running the test suite takes up about five minutes, which is way too long.

Part of that is down to the choice to write full integration tests instead of unit tests (and thus hitting both an actual SQLite database and an actual MySQL database in the tests) but I also think that there is a lot to be gained by optimizing.

For example, I'm running the tests in separate processes. I don't think this is entirely necessary, even if it might require some changes to be able to run them all in the same process.