gtphub: fix User plane decoding, add unit test.

Split decoding return code GTP_RC_PDU in GTP_RC_PDU_C and GTP_RC_PDU_U.
Don't do IEs in GTP_RC_PDU_U.

Add a unit test for User plane data, expected to fail (nonstandard port case).

In gtphub_test.c, tweak logging so that it is easily visible which test
produced which output. Also add the global resolved_sgsn_addr and ggsn_sender,
symmetrically to resolved_ggsn_add and sgsn_sender.

Sponsored-by: On-Waves ehi
diff --git a/openbsc/tests/gtphub/gtphub_test.ok b/openbsc/tests/gtphub/gtphub_test.ok
index eac6f0c..24b9aad 100644
--- a/openbsc/tests/gtphub/gtphub_test.ok
+++ b/openbsc/tests/gtphub/gtphub_test.ok
@@ -1,2 +1,9 @@
-Wrap: returning GGSN addr from imsi 240010123456789 ni internet: 192.168.43.34 port 434
+test_echo
+test_create_pdp_ctx
+- __wrap_gtphub_resolve_ggsn_addr():
+  returning GGSN addr from imsi 240010123456789 ni internet: 192.168.43.34 port 2123
+test_user_data
+- __wrap_gtphub_resolve_ggsn_addr():
+  returning GGSN addr from imsi 240010123456789 ni internet: 192.168.43.34 port 2123
+- user data starts
 Done