Enable parallel forks for tests

[koperagen]

Let's see how it affects build server performance. I tried to benchmark locally, but execution time is just way too unstable to tell if it actually helps or not Configured according to https://docs.gradle.org/current/userguide/performance.html#execute_tests_in_parallel

[Jolanrensen]

That is a good idea!

[Jolanrensen]

I converted the PR to a draft for now since we didn't get the results we wanted (yet). Feel free to change it if it improves :)

[koperagen]

I think we have a better chance of improving performance, after we eventually get rid of gradle integration tests