[Proposal] Make tests run in multiple processes

[pascal-roth]

Proposal

Make tests run according to the following proposal and possibly get a speedup.

Checklist

• [ ] I have checked that there is no similar issue in the repo (required)

Acceptance Criteria

• [ ] Check if speedup can be achieved

[RandomOakForest]

Thanks for posting this. I'll tag this as an enhancement as well.