Refactor test_task_retry 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 test_task_retry tests.

Partially implements: blueprint mistral-workflow-start-task

Change-Id: I3e7a2a09d7a17cf7924e678dad78a0c12bd7b03a
Merge "Refactor test_task_retry to not rely on start_task"