Add network tests

[iphydf]

Currently, the tests only cover some simple functionality that can be tested without starting a tox network. The test suite should also contain auto-tests that check whether callbacks receive the right data and all the functions are called correctly (e.g. no segfault or other error).

https://travis-ci.org/TokTok/js-toxcore-c/builds/164348802 shows that the tests pass although the API changed considerably, and any attempt to actually run anything with this library would fail.