stp: Don't expect a fixed SLS in IPA -> M3UA case

We are about to introduce a feature for SLS "randomization" in osmo-stp,
so let's modifiy our STP tests to permit any SLS in the M3UA messages
after IPA -> M3UA conversion.

Change-Id: Ib70322b80a72e3e7dc3155bca9e67b03efb413e0
Related: SYS#6543
1 file changed