Improve test suite speed

[TeaDrinkingProgrammer]

The test suite is on the slow side, so I want to see whether I can make some minor changes to speed it up.

[nhoening]

That is of course very welcome! Curious what you find.

We divided tests into the ones we want to have a fresh database, and others who are okay without (e.g. just reading). I believe the former set eats most of the time.