Changes

asterisk: Introduce test TC_ims_reregistration

So far Asterisk seems to be doing "initial registration" during
re-registration, instead of using the existing security context.
This is "OK" afaiu, but not the best pick (TS 24.229 5.1.1.4.1).

Related: SYS#6983
Change-Id: Icaeec6e2be3469b9e49d4361ca2b039ab44683c0