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