Reuse containers in tests

[provokateurin]

Currently we spawn a new container for every single test to ensure a clean state and no side effects. This slows down our tests because significant time is spent on spawning the containers.
It should be possible to migrate some of the tests to share containers. It is not possible to do this everywhere but we are also not forced to share them everywhere.
I hope this will especially boost the performance of the news tests as they are our slowest right now and I think a lot of that has to do with re-fetching the feeds in every test which slows them down massively.

[provokateurin]

I haven't migrated all tests yet, but there already is a huge speed benefit. The tests mostly execute as fast as normal unit tests would. Haven't checked news yet, but I'm sure it will be way more performant.

[provokateurin]

I'm using random shuffling of the tests to find problems and we might want to do this in the CI as well with a new random seed every time.