s390x: smp: Add restart when running test

Let's make sure we can restart a cpu that is already running.
Restarting it if it is stopped is implicitely tested by the the other
restart calls in the smp test.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20200429143518.1360468-10-frankja@linux.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>