sgsn/test: Add test that intercepts gprs_gsup_client_send

This test replaces gprs_gsup_client_send by a custom function, that
emulates a GSUP remote peer by calling gprs_subscr_rx_gsup_message
with responses for all requests. It then executes a full
Attach/Detach cycle.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/sgsn/sgsn_test.ok b/openbsc/tests/sgsn/sgsn_test.ok
index c9c0165..cff2920 100644
--- a/openbsc/tests/sgsn/sgsn_test.ok
+++ b/openbsc/tests/sgsn/sgsn_test.ok
@@ -13,6 +13,7 @@
 Auth policy 'remote', triplet based auth: Testing GMM attach
 Auth policy 'remote', GSUP based auth: Testing GMM attach
 Auth policy 'remote', GSUP based auth: Testing GMM attach with retry
+Auth policy 'remote', real GSUP based auth: Testing GMM attach
 Testing GMM reject
   - Attach Request (invalid MI length)
   - Attach Request (invalid MI type)