MGCP_Test: support multiple codecs

At the moment The RTP emulation and MGCP_Test only allow to specify one
codec and one set of RX/TX fixed payload octet strings to verify against.

This is quite limiting since it might be necessary to test against
different types and formats of payloads simultaneously in order to see
if osmo-mgw converts or forwards them correctly.

Let's extend this to support multiple codecs on MGCP/SDP level plus
support for multiple RTP payloads on RTP emulation level.

Related: OS#5461
Change-Id: I8422313fccad1bfcee52c933f643068bebdaf2d5
4 files changed