[UnblockOnMainActorTest] adjust workload to log level

The UnblockOnMainActorTest regularly timeout in our CI which looks to be a regular timeout because I could easily reproduce the timeout on fauibig03.

Hopefully this change fixes regular timeouts like https://gitlab.cs.fau.de/aj46ezos/emper/-/jobs/506757 in the CI and only leaves real faulty timeouts.