Testrunner: Take default only when no other variant is selected

If a variant is selected either by setting the environment flag or
passing the variant as an arg to the python testrunner, the default
variant of that type will not be used unless it is explicitly set
by the above methods.

Test: Checked the behavior by running locally.
Change-Id: I6f480249408c0bf406b660e5098f298f21ff2db4