Autobahn integration

This MR integrates the autobahn test suite into CI. Autobahn is a testsuite for testing websockets (either as a server or as a client). The changes are the following:

• Implementation of a testing client and tooling for serving autobahn tests (done by Claudio).
• Integration of autobahn in the CI container. Tests are served from a virtual environment (as described in https://github.com/crossbario/autobahn-testsuite).

What's missing is a means to run the tests automatically (basically a step that will launch the script tests/autobahn/autobahn-server.sh and run soup-autobahn-test-client).