GERAN: allow GSM SRES on UMTS AKA challenge

Store the established security context type (GSM or UMTS) instead of the
boolean flag is_authenticated. Provide the previous boolean query with thin
sgsn_mm_ctx_is_authenticated() function.

Knowing which security context was established will be necessary for OS#3224,
i.e. using the proper ciphering key, which is not yet tested properly, and
probably not correct at this stage.

This change will make new SGSN_Tests.TC_attach_umts_aka_gsm_sres pass.

Related: OS#3193 OS#3224
Change-Id: I36807bad3bc55c0030d4f09cb2c369714f24bec7
diff --git a/tests/sgsn/sgsn_test.c b/tests/sgsn/sgsn_test.c
index df4df3b..0e5267b 100644
--- a/tests/sgsn/sgsn_test.c
+++ b/tests/sgsn/sgsn_test.c
@@ -1132,7 +1132,7 @@
 {
 	/* Fake an authentication */
 	OSMO_ASSERT(mmctx->subscr);
-	mmctx->is_authenticated = 1;
+	mmctx->sec_ctx = OSMO_AUTH_TYPE_GSM;
 	gprs_subscr_update_auth_info(mmctx->subscr);
 
 	return 0;