sgsn: Add test that exposes a dangling pointer to the LLME

On detach the LLME get's unassigned (and hence destroyed) but the
GMM context will still point to that dead structure.
diff --git a/openbsc/tests/sgsn/sgsn_test.ok b/openbsc/tests/sgsn/sgsn_test.ok
index 045b9e5..57326fa 100644
--- a/openbsc/tests/sgsn/sgsn_test.ok
+++ b/openbsc/tests/sgsn/sgsn_test.ok
@@ -1,2 +1,3 @@
 Testing LLME allocations
+Testing GMM detach
 Done