Add tests which check task dependencies

- Added new test file with tests which check workflow execution and
task dependencies.
- Update base.py file, rename '_create_execution' method, make it more
common
- Refactor tests in the test_mistral_base file according method remaning
- Change test workflow demo.yaml

Targets blueprint: mistral-workflow-execution-tests

Change-Id: If58d9d0da8a7204214f92fdb12ed87cb1801c17b
Merge "Add tests which check task dependencies"