gbproxy/test: Add expect_msg checks to test_gbproxy_keep_info

This patch adds assertions to check the LLC/GMM message received from
the gbproxy by the test framework within the function
test_gbproxy_keep_info.

It also fixes the source address of the DETACH ACC messages.

Sponsored-by: On-Waves ehf
gbproxy: Patch BSSGP P-TMSI in PAGING PS messages (TODO)

Currently the P-TMSI IE in PAGING_PS is not patched.

This commit adds code the patch BSSGP P-TMSI IE in
gbproxy_patch_bssgp independently from the P-TMSI patching at the LLC
layer.

TODO:
  - finish test case

Sponsored-by: On-Waves ehf
sgsn: Added TODOs (TODO)

TODO:
  - fix and remove TODOs
  - add test cases for Detach(reattach)
  - add test cases for PDP context deletion (OW#1311)
  - remove this commit

Sponsored-by: On-Waves ehf