Changes

ttcn3-bts-test/osmo-bsc.cfg: do not enable CBCH slots by default

As it turns out, test cases from BTS_Tests_SMSCB configure the
CBCH slots "on the fly" depending on the test case parameters.
This configuration also makes some test cases fail, so let's
(re)configure SDCCH8+CBCH timeslots to SDCCH8.

Change-Id: I3699450542dc1350971fbf87ad60a330428dec27