s390: Add testcase for the SRNMT instruction.
Patch by Maran Pakkirisamy (maranp@linux.vnet.ibm.com).
Part of fixing BZ 307113.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13325 a5019735-40e9-0310-863c-91ae7b9d1cf9