Websocket: move protocol tests to the new TestMessagingProtocol class

When we test our websocket transport, we test two types of things:
1. Our websocket API
2. Our websocket driver, factories, protocols

I think 'MessagesBaseTest' should contain only websocket API tests, but I see now that it contains also protocol-related tests, for example, 'test_invalid_request':
See https://github.com/openstack/zaqar/blob/master/zaqar/tests/unit/transport/websocket/v2/test_messages.py#L596

This test case (and maybe some others) can be put in the newly introduced 'TestMessagingProtocol' class:
See https://github.com/openstack/zaqar/blob/master/zaqar/tests/unit/transport/websocket/test_protocol.py