Remove grisp_manager from tests

[maehjam]

Instead of using grisp_manager we use a test server grisp_connect_test_server that forwards all websocket traffic to listening processes (that called grisp_connect_test_server:listen()). This way we can check what grisp_connect sends over the websocket and control the response it receives in the test case.

Note 1: For now I kept using ssl (that was already implemented) in the test. We can remove it for testing in a separate PR.

Note 2: I only adjusted the existing test cases. They do not cover much.

[sylane]

I commented on things I feel would be better, but feel free to have a talk and shot my comments down :)