ggsn: Fix TC_pdp4_act_deact_with_single_dns()

In TC_pdp4_act_deact_with_single_dns we activate, deactivate and then
re-activate a PDP context. Hoewver, we re-use the same variable and
don't reset the state in between.  This results in the second PDP CTX
activation to include an end-user-address (static IP allocation), which
OsmoGGSN doesn't implement.

Before osmo-ggsn Change-Id Iac8868438655fe4e5e07d167d7dbd6273dbb7678,
the test passed as osmo-ggsn simply ignored the requested static address.
After that change, we reject static addresses and hence the test starts
to fail.

Change-Id: I1b1869bc2cee39c8fddd8fa63f48bdaa6a65e462
Related: OS#5097
diff --git a/ggsn_tests/GGSN_Tests.ttcn b/ggsn_tests/GGSN_Tests.ttcn
index be0e565..a0c7b16 100644
--- a/ggsn_tests/GGSN_Tests.ttcn
+++ b/ggsn_tests/GGSN_Tests.ttcn
@@ -1147,6 +1147,7 @@
 		f_pdp_ctx_del(ctx, '1'B);
 
 		/* PCO with secondary DNS only */
+		ctx := valueof(t_DefinePDP(f_rnd_imsi('26242'H), '1234'O, c_ApnInternet, valueof(t_EuaIPv4Dyn)));
 		ctx.pco_req := valueof(ts_PCO_IPv4_SEC_DNS_IPCP);
 		f_pdp_ctx_act(ctx);
 		pco_neg_dns := f_PCO_extract_proto(ctx.pco_neg, '8021'O, 1);