Allow emergency calls without IMSI (using IMEI as MI)
This is supported since a few moths ago in osmo-bsc, see https://osmocom.org/issues/5849
However, this seems to be not the case yet for osmo-msc.
According to our TTCN3 MSC_Tests, we right now have these tests passing:
Which seems to describe that we are currently rejecting emergency calls with IMEI.
We should add a new TC_emerg_call_imei test (or change TC_emerg_call_imei_reject) so that we can test emergency calls without IMSI, and implement whatever is needed in osmo-msc to have it passing.