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.
