s390/test_unwind: convert to KUnit

Modified stack unwinder self tests to use kunit framework. The
functionality stayed the same but the output format is now in tap13
format.

Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Mete Durlu <meted@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>