SGSN_Tests: fix TC_attach_timeout_after_pdp_act

This patch fixes several problems:

* missing "vc_conn.done": the actual testsuite logic, which can be
  found in f_TC_attach_timeout_after_pdp_act(), was never executed
  fully because we did not wait for the BSSGP_ConnHdlr to complete;
* too short Tguard value: the testsuite logic takes slightly more
  time to complete than the default timeout of 30.0 seconds;
* osmo-sgsn does not require authentication for the second ATTACH.req:
  the testsuite logic gets stuck in f_gmm_auth() after sending the
  second ATTACH.req because:
** osmo-sgsn is waiting for a response to GMM IDENTITY REQUEST,
** osmo-sgsn does not send GSUP SendAuthInfo.req again.

As can be seen from the test execution artifacts on Jenkins, this
testcase never passed: either failing due to an error, or declaring
no verdict at all.  The average execution time is 650 ms.

Change-Id: Ibaf2134247153471bd45d7a7f91155294c6c6de5
Fixes: 3ede9e32b "sgsn: Add TC_attach_timeout_after_pdp_act"
Closes: OS#4221
---
M sgsn/SGSN_Tests.ttcn
1 file changed, 46 insertions(+), 5 deletions(-)