add DTX fill frame BTS tests

Add tests TC_tch_sign_l2_fill_frame and TC_tch_sign_l2_fill_frame_dtxd.

TC_tch_sign_l2_fill_frame is already passing and verifies that fill
frames are sent if there is nothing else to transmit on a SDCCH4/SDCCH8,
TCH/H, or TCH/F signalling channel where DTX is disabled for downlink.

TC_tch_sign_l2_fill_frame_dtxd is currently failing. It verifies that
only specific fill frames are sent, as required by GSM 05.08 for TCHF
signalling channels with DTX enabled for downlink. At present, our
implementation generates no fill frames in this case, which is one
piece of the problem described in issue OS#1950.

Change-Id: Id4e0de6e78b62cd408f600a57a28617d91da64af
Related: OS#1950
3 files changed