ggsn: Disable GTP-U tx sequence number by default

GTP-U transmit sequence numbers are entirely optional and probably
don't serve any real purpose in real-world deployments.

While OsmoGGSN in userspace implements support for it, the kernel GTP-U
implementation doesn't.  This means a number of tests fail against
kernel GTP-U only for that reason.

Let's switch all tests to disable GTP-U sequence numbers, and only
enable it in one specific test.  This way, we can execute the tests
also against kernel GTP-U.

Related: OS#3215
Change-Id: I666f5276749ef6a1a4dc170a3b9a747f626f6b2c
diff --git a/ggsn_tests/GGSN_Tests.ttcn b/ggsn_tests/GGSN_Tests.ttcn
index cb6aabf..d099681 100644
--- a/ggsn_tests/GGSN_Tests.ttcn
+++ b/ggsn_tests/GGSN_Tests.ttcn
@@ -93,7 +93,7 @@
 		var uint16_t g_d_seq_nr;
 
 		port TELNETasp_PT GGSNVTY;
-		var boolean use_gtpu_txseq := true;
+		var boolean use_gtpu_txseq := false;
 	}
 
 	private function f_init_vty() runs on GT_CT {