Changes

sgsn: add TC_attach_gmm_attach_req_while_gmm_attach_changed

Add a TC to to simulate multiple GMM Attach with a different
content.
When receiving a GMM Attach while in an Attach, but with a different
message, the old GMM Attach should be discard silently and a new
GMM Attach procedure should be initiated.

Change-Id: Ia721d3c29bf09014b197670ae02d1cd1901750bc