KVM: s390: selftest: memop: Add cmpxchg tests

Test successful exchange, unsuccessful exchange, storage key protection
and invalid arguments.
Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
Acked-by: Janosch Frank <frankja@linux.ibm.com>
Link: https://lore.kernel.org/r/20230207164225.2114706-1-scgl@linux.ibm.com
Message-Id: <20230207164225.2114706-1-scgl@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>