Changes

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