fix TC_pdp4_act_deact_with_separate_dns

Do not delete the PDP context too early, and look for the second DNS
server in the correct place (2nd match on IPCP protocol, not 1st).

Update a comment which talks about a bug which has been fixed.

Change-Id: I109491cc9ccb060792e29bf6b2999ef48723edbf
Related: OS#3319
Related: OS#3381
diff --git a/ggsn_tests/GGSN_Tests.ttcn b/ggsn_tests/GGSN_Tests.ttcn
index 6ae205f..3c99fd9 100644
--- a/ggsn_tests/GGSN_Tests.ttcn
+++ b/ggsn_tests/GGSN_Tests.ttcn
@@ -1017,11 +1017,10 @@
 		if (not match(pco_neg_dns, pco_neg_dns_expected)) {
 			setverdict(fail, "Primary DNS IPv4 PCO option not found");
 		}
-		f_pdp_ctx_del(ctx, '1'B);
 
 		/* Check if secondary DNS is contained */
-		/* Note: this check fill fail due to a bug in osmo-ggsn, see also OS#3381 */
-		pco_neg_dns := f_PCO_extract_proto(ctx.pco_neg, '8021'O, 1);
+		/* This used to fail due to a bug in osmo-ggsn, see OS#3381 */
+		pco_neg_dns := f_PCO_extract_proto(ctx.pco_neg, '8021'O, 2);
 		pco_neg_dns_expected := '0200000A8306'O & ggsn_ip4_dns2
 		if (not match(pco_neg_dns, pco_neg_dns_expected)) {
 			setverdict(fail, "Secondary DNS IPv4 PCO option not found");