Changes

stp: Fix brokeness in TC_tmt_loadshare

The test was not even setting the traffic-mode in the UIT.
Furthermore, it was expecting pure round-robin behavior, which was the
older behavior of osmo-stp when loadshare traffic-mode was selected.

Actually split the test into 2, naming them properly (since round robin
is not a AS traffic mode in itself, but a possible implementation of the
loadshare traffic-mode.

The new test validates the usual loadshare traffic-mode based on SLS
distribution.

Related: SYS#7112
Depends: libosmo-sigtran.git Change-Id I61340549c596f1c04bc2269dbc165c327bf72037
Change-Id: I16d81cb2f88bb2927f248182ad4f8f27c8c24859