Changes

tests: add unit tests for osmo_rtp2trau()

The suite of unit tests added in this patch exercises osmo_rtp2trau()
followed by osmo_trau_frame_encode() for the following combinations
of speech codec and TRAU frame type:

* FR and EFR codecs, TRAU-16k frames, both DL and UL
* HRv1 codec, TRAU-8k format, DL only

(RTP->TRAU conversion for HRv1 does not currenty support TRAU-UL
 output.)

All output TRAU frames contained in rtp2trau_*.ok files (expected
output for unit tests) have been manually checked with Themyscira
decoding utilities trau-parse-hex (16k) and trau-hr-dump-hex (8k);
these utilities in turn were developed to analyze TRAU frame streams
captured from historical BTS and TRAU equipment, hence the chain of
truth reference comparison is traceable to pre-existing historical
hardware implementations of GSM 08.60 and 08.61 specs.

Change-Id: Ia5ca8af6bd3a899253bbcc718b70e43f2265b495

rtp2trau HR: add support for TRAU-8k-UL frame output

When osmo_rtp2trau() function is used to pass traffic to an E1 BTS,
it only needs to generate TRAU-DL frames, not TRAU-UL.  OTOH,
ability to generate TRAU-UL frames is needed for TFO, i.e., for
software implementation of TFO-capable speech transcoders.
osmo_rtp2trau() already supports TRAU-UL frame output for FR and
EFR codecs, but not for HR codec in TRAU-8k framing - add the
missing support.  (Out of the two possible TRAU frame formats for
GSM-HR codec, TFO always uses TRAU-8k format.)

Change-Id: I288cfa7d467125f7732ef1d0ef83b933e41da536