sgsn/test: Add test case for Detach Request (MO, power-off = 1)

Currently only a Detach Request (MO) message with power_off = 0 is
checked.

This commit adds a new test case with power_off set to 1. It also
adds checks for the number of messages generated by the SGSN to
verify that these messages are handled differently.

Note that the handling of power_off isn't implemented yet. Therefore
the corresponding assertion is being disabled yet.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/sgsn/sgsn_test.ok b/openbsc/tests/sgsn/sgsn_test.ok
index 57326fa..4c475d9 100644
--- a/openbsc/tests/sgsn/sgsn_test.ok
+++ b/openbsc/tests/sgsn/sgsn_test.ok
@@ -1,3 +1,4 @@
 Testing LLME allocations
 Testing GMM detach
+Testing GMM detach (power off)
 Done