Refactor test_engine to not rely on start_task

Some tests rely on starting same workflow at various tasks.
This wont' work once start task is defined at workflow level.
Fixing tests as a first step.

Also, separated the samples for direct flow (on-success/etc)
and reverse flow (with 'requires'), towards deprecating
'require' based flows (deprecate-requires-in-dsl)

Partially implements: blueprint mistral-workflow-start-task

Change-Id: Id8d35962878ee9edf6b8de9cdbe0af779c7427aa
Merge "Refactor test_engine to not rely on start_task"