PCU_Tests_RAW.ttcn: add a test case for continuous Timing Advance control

Unlike the circuit-switched domain, Uplink transmissions on PDCH time-slots
are not continuous and there can be long time gaps between them. This happens
due to a bursty nature of packet data. The actual Timing Advance of a MS may
significantly change between such rare Uplink transmissions, so GPRS introduces
additional mechanisms to control Timing Advance, and thus reduce interference
between neighboring TDMA time-slots.

At the moment of Uplink TBF establishment, initial Timing Advance is measured
from ToA (Timing of Arrival) of an Access Burst. This is covered by another
test case - TC_ta_rach_imm_ass. In response to that Access Burst the network
sends Immediate Assignment on AGCH, which _may_ contain Timing Advance Index
among with the initial Timing Advance value. And here PTCCH comes to play.

PTCCH is a unidirectional channel on which the network can instruct a sub-set
of 16 MS (whether TBFs are active or not) to adjust their Timing Advance
continuously. To ensure continuous measurements of the signal propagation
delay, the MSs shall transmit Access Bursts on Uplink (PTCCH/U) on sub-slots
defined by an assigned Timing Advance Index (see 3GPP TS 45.002).

The purpose of this test case is to verify the assignment of Timing Advance
Index, and the process of Timing Advance notification on PTCCH/D. The MTC
first establishes several Uplink TBFs, but does not transmit any Uplink
blocks on them. During 4 TDMA multi-frame periods the MTC is sending RACH
indications to the PCU, checking the correctness of two received PTCCH/D
messages (period of PTCCH/D is two multi-frames).

At the moment of writing, PTCCH handling is not implemented in OsmoPCU
(neither PTCCH/D messages are correct, nor PTCCH/U bursts are handled).

Additionally, this change introduces a new message type, which is used
for sending commands to the RAW components - RAW_PCU_Command. Commands
can be used to (re)configure components at run-time.

Change-Id: I868f78e3e95a95f8f2e55e237eea700d7d4726a3
Related: SYS#4606

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