kprobes: add testcases for s390

Add a few testcases to make sure that it's not possible to place
a kprobe in the mid of an instruction on s390.

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