BSC_Tests: add TC_chan_alloc_algo_{a,de}scending

The aim of these new test cases is to verify both ascending and
descending modes of the BSC's channel allocator.  Both tests are
using BTS2, which has 4 TRX instances.

The common test scenario can be described as follows:

1. Establish an SDCCH channel, send some dummy payload on it.
2. Send a BSSMAP Assignment Request for a TCH/F channel.
3. Expect RSL CHANnel ACTIVation for a TCH/F channel.
4. Respond with RSL CHANnel ACTIVation NACK (*).
5. Expect a BSSMAP Assignment Failure.

These steps are repeated several times.  Note (*) that for the
sake of simplicity, I am aborting the assignment procedure by
sending a NACK, so that the requested logical channel becomes
BORKEN and behaves like an occupied one until the A-bis/OML
link is re-established.  This reduces the overall complexity.

Change-Id: Ic74a9cd264320a9a17648f9331b67fb2c2404122
Related: SYS#5460
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M bsc/BSC_Tests.ttcn
1 file changed, 66 insertions(+), 0 deletions(-)