s390x: Fix stsi unaligned test and add selector tests

Alignment and selectors test order is not specified and so, if you
have an unaligned address and invalid selectors it's up to the
hypervisor to decide which error is presented.

Let's add valid selectors to the unalignmnet test and add selector
tests.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>