qtqa: Make flaky test mode configurable

There has been a request to disable flaky test plugin. We should
rely the result of initial test run.

Change-Id: I8fa62873ee94b5265d3c572dd734e8fbe451140d
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>