Qtqa: Configurable script instead of default ci testrunner

Some mobile platforms can't be tested with default CI test setup.
Instead some of them, like Android Qt port is relying on its own
testrunner script. To be able to utilize this in CI, we need a way
to run non-default testrunners as well.

Task-number: QTQAINFRA-641
Change-Id: I8f59c20d1fc3fbe204f3ac624aad056e68e91c41
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>