KVM: s390: selftests: enable some common memory-related tests
Enable the following tests on s390:
* memslot_modification_stress_test
* memslot_perf_test
* mmu_stress_test
Since the first two tests are now supported on all architectures, move
them into TEST_GEN_PROGS_COMMON and out of the indiviual architectures.
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>