Add tests for ToxAv

[iphydf]

Currently the Alice and Bob tests for ToxAv are disabled, because the A/V subsystem changed its pull model to a push model. The A/V tests currently assume a pull model.

Tasks:

• Ensure that all errors work correctly.
• Change the A/V tests to work with the push model.
• Add more tests for changing bit rates, pausing and resuming audio.
• Add video tests, including pause/resume and different bit rates.
• Remove the dependency on a human ear or eye for testing. Use some simple audio fingerprinting or fuzzy compare function to test A/V correctness.
• Add a stutter test that creates a suboptimal network situation (e.g. via Tor) and checks that the A/V core tunes down the bit rate correctly.
• Also add an inverse test of the above: check that connecting to localhost tunes up the bit rate to the maximum.