pcu: Test Downlink UNIT_DATA messages from SGSN containing IMSI IE

osmo-pcu.git 0052051c07af63da98137c9f8e3b3119642eb587 introduced a bug
(fixed in 1d68cdff928f32941aff36c70e4543203c283d15) where no MS could
GMM attach to the network, but no test showed the issue.
This couple test verifies both correct behavior and also ensures wrong
IMSI is detected and reported.

Related: OS#4729
Change-Id: I5072d80b7ed9945a6083cdf3254efb8b8f119c54
2 files changed