Changes

pcu: Fix dummy DL block received due to timer race conditions

Timer X2002, which manages delay at PCU between sending DL TBF Ass over
CCCH and start transmitting for it over PDCH, is clock-time based.
As a result, timer at PCU process and ttcn3 process may time out
slightly differently. Hence, it can happen that we request a DL block
immediatelly *before* the timer triggers at the PCU. In that scenario,
PCU transmits a dummy block instead of a data block.
Account for this race condition in several tests; some tests already
used this formula.

Change-Id: Ic2bb953483cd1f678cbe32c7a94e177f79ecb4e4