sgsn: Create testcase that verifies that llmes get deleted

On an "unassignment" this code verifies that the LLME will vanish
from the list of LLMEs. We assume that this doesn't create a
memory leak.
diff --git a/openbsc/tests/sgsn/sgsn_test.ok b/openbsc/tests/sgsn/sgsn_test.ok
index e69de29..045b9e5 100644
--- a/openbsc/tests/sgsn/sgsn_test.ok
+++ b/openbsc/tests/sgsn/sgsn_test.ok
@@ -0,0 +1,2 @@
+Testing LLME allocations
+Done