ttcn3-bts-test: use an older version of fake_trx.py
The latest version of fake_trx.py [re-]introduces burst queueing [1], making its behavior more closely resemble that of a real transceiver. However, this change has also introduced stability issues, leading to increased flakiness in ttcn3-bts-test.
As a temporary workaround, use an older version predating this change. Unset the "--sched-rr-prio" [2], which is not supported by that version.
Change-Id: I251082474bb29d0169f88b5bd545c97d487c8353 Related: [1] osmocom-bb.git I41291708effdd2c767be680fff22ffbd9a56815e Related: [2] 6374a9e ("ttcn3-bts-test: Run fake_trx with sched-prio-rr") Related: OS#6672
