Engine tests refactoring

This change makes tests from test_action_engine.py more focused and
deterministic:
- replace assertIsSubset with assertIsSuperAndSubsequence, which checks
  order, too;
- remove all sleeping from test tasks, it does not help anything anyway;
- refactor tests that verify behaviour in case of task failing in nested
  subflow.

Change-Id: I87ce1e5eed4a3a93c3c6593b618e82cfdd68204f