Avoid moving DL-TBF from old_msg to new_ms during ms_merge

The DL-TBF assigned to another MS object may have a totally different
set of reserved resources (TS set, TRX, etc.), so one cannot simply move
those to the new MS. To start with, if the 2 MS are on different TRX it
is clear that one of them will not be really in operation. That's most
probably the DL-TBF being in ASSIGN state on CCCH waiting for PCUIF_CNF
and later X2002 to trigger to start sending DL blocks, but without
confirmation whether the MS is really there. Since the other new MS
object probably has a UL-TBF, that's the one probably operative, and
hence a new DL-TBF can be created at that same time and assigned through
UL-TBF's PACCH.

Unit test test_ms_merge_dl_tbf_different_trx showcases the above
scenario.

Related: SYS#6231
Related: OS#5700
Related: 677bebbe5c49d4607322e96053fe14ddd78d9549
Change-Id: Ie8cb49d5492cfc4cbf8340f3f376b0e6105e8c82
---
M src/gprs_ms.c
M src/tbf_dl_fsm.c
M tests/tbf/TbfTest.cpp
M tests/tbf/TbfTest.err
4 files changed, 98 insertions(+), 61 deletions(-)