Changes

msc: add/expect network-originated MT-Forward-SM.Err

The idea is to have two variants of the MT-Forward-SM.Err:

* _MS: originated by the MS/UE (via RP-ERROR),
* _NET: originated by the network (MSC) itself.

In both testcase scenarios we expect the network to indicate the
MT_FORWARD_SM_ERROR on its own, due to the lack of response from MS/UE.
Use the right template kind for that, expect a specific Cause value.

Change-Id: Ic27d943645fdff8d34f958e3bf16c8ee6e8368d2
Related: osmo-msc.git I51d92752471147e6d21a5059bebb0702b32642a5
Related: e9de1515 ("msc: add TC_gsup_mt_sms_{paging,conn}_timeout")
Related: SYS#7323