ggsn: Speed up tests by lowering retransmission T3,N3 timers

The params are set in GGSN_Tests.cfg and not in GGSN_Tests.default,
because those lower values are not really good default ones as per spec.
For instance when running against open5gs-smfd, these values should be
left to the default in code of 5 and 3 seconds, because open5gs-smfd
doesn't support changing the values through cfg file yet.

Related: OS#5485
Change-Id: I3798fba89c2c357afeaa83a73759442c6c433cea

ggsn_tests/GGSN_Tests.cfg

@@ -13,6 +13,10 @@
 [TESTPORT_PARAMETERS]
 
 [MODULE_PARAMETERS]
+# local T3-RESPONSE timeout, seconds:
+GGSN_Tests.mp_t3_response := 1;
+# local N3-REQUESTS counter:
+GGSN_Tests.mp_n3_requests := 2;
 
 [EXECUTE]
 GGSN_Tests.control

ggsn_tests/osmo-ggsn.cfg

@@ -45,6 +45,9 @@ line vty
 ggsn ggsn0
  gtp state-dir /tmp
  gtp bind-ip 127.0.0.2
+ timer gtp T3 1
+ timer gtp T1003 2
+ timer gtp X3 2
  apn internet
   gtpu-mode tun
   tun-device tun4