bts: Refactor code to support mixed DEFAULT + NORMAL and EXTENDED CBCH

The existing code structure could only test for normal messages with a
NULL default, but didn't handle situations where normal and/or schedule
messages were superimposed on top of DEFAULT messages.

Also, prepare the infrastructure for testing both CBCH BASIC and CBCH
EXTENDED.

No new tests are introduced, the code should behave identical before
and after this patch.

Change-Id: I144c7d833b79c648b1ac69a6155f3603025ede5c
Related: OS#4011
2 files changed