CI improvements & speedups

[stenczelt]

problems

The CI is very slow:

• pre-commit takes 4m (no speedup from caching)
• unit-tests take over 20 minutes

Looking at the test suite, there are not all "unittests" per se, but include a lot of integration/e2e etc. test as well. This is very good, however confusing and more difficult for local development.

proposed changes

1. speed up the pre-commit CI job
2. categorise tests and separate fast and slow ones