s390/test_unwind: add WARN if tests failed

Trigger a warning if any of unwinder tests fail. This should help to
prevent quiet ignoring of test results when panic_on_warn is enabled.

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>