scsi: ufs: add support for test specific completion check

Introduce a new callback 'check_test_completion_fn' to test-iosched
framework. This callback is necessary to determine if a test has
completed or not in situation where the request queue is empty, but the
test was not completed.

Change-Id: I60bd8cccffacab11a5a7cba78caccf53fea3e1d8
Signed-off-by: Dolev Raviv <draviv@codeaurora.org>