bts/BTS_Tests.ttcn: update TC_rach_max_ta test case

Since the I662294fe3136cf7a259be13816a3e63f7db9a948, OsmoBTS
should pass RACH requests with ToA > -2 symbol periods only.

We do allow early signal arrival up to 2 symbols, otherwise
it is most likely noise, interference or a ghost.

Change-Id: Icccc88545ed3aabd6da28a40599a8a77d1de477d
1 file changed