Remove redundant 'import testscenarios' from tests

In review [1], Angus added testscenarios support in BaseTestCase. So we don't
need to import the same anymore in other modules.

[1] https://review.openstack.org/#/c/91711/

Change-Id: I7883b68226dc20a5ce89426ad8259d69d8050531
Merge "Remove redundant 'import testscenarios' from tests"