add tests around connectivity issues

[matehat]

Test that the following scenarios work:

1. Connection starts dead, socket connect is initiated, connection is restored, socket connect succeeds
2. Connection starts as usual, socket connection is established, connection drops, channel join is attempted, connection is restored, channel join succeeds
3. Connection starts as usual, socket connection is established and channel joined, connection drops then is restored, message is sent on channel and reply is received

To test those, a control side-channel was added to the test backend so the client can tell the backend to start and stop the websocket server at precise times.