s390x: skrf: Add exception new skey test and add test to unittests.cfg

When an exception new psw with a storage key in its mask is loaded
from lowcore, a specification exception is raised. This differs from
the behavior when trying to execute skey related instructions, which
will result in special operation exceptions.

Also let's add the test to unittests.cfg so it is run more often.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20200807111555.11169-3-frankja@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>