Refactored the way how testscenarios tests are run

Instead of using custom loader we now use a base class from
testscenarios. This allows us to retain compatibility with
unittest test runner and run a single test if needed.

Change-Id: Id5ae4fd0edfa868aaf6a57bd2e5f212b51b722e1
Merge "Refactored the way how testscenarios tests are run"