pcu: TC_mt_ping_pong: Request UL TBF on last DL ACK

Let's test the code path where UL TBF is requested through DL ACK/NACK
here, since we already test the usual UL TBF through CCCH approach in
most tests.

rlc_mode is changed to ACKED since that's the mode we are using so far
in tests.

Change-Id: I5a9a2e8107c87fdbf74cc2f09ae5eeafbb13ad55
3 files changed