Deal with flaky tests

[vpavkin]

There are two issues with tests, which make them fail on random rare occasions:

1. timeouts.
   If the runner is slow, then some actors in the tree may not answer within timeout.

2. actor count and non-deterministic sometimes.
   This is more weird, but for some reason sometimes actor system creates more actors than usual. It's inherently problematic, since we're digging into akka internals here. Not sure what can be done about it.