Unit tests for parser & composer status codes

[gavv]

This is a follow-up for #736 and #737.

Add simple tests to check how status codes returned from implementations of IParser and IComposer:

• Add unit tests for rtp::Parser, rtp::Composer, fec::Parser, fec::Composer for the cases when they return non-OK status.

• Add unit tests for components that use parsers and composers. In tests, check that the component properly forwards error code from parser/composer to the caller.

  For example, a test makes Composer to fail with StatusNoMem, and check Shipper::write() returns StatusNoMem.

  Classes to cover:

  • fec::BlockReader
  • fec::BlockWriter
  • packet::Shipper
  • audio::Packetizer
  • audio::Depacketizer