Dangling Websocket sessions throw exceptions after timeout

As reported in #338:

> The next thing: I want to close them [Websocket sessions] because I don't want old sessions hang. Also after some time they start to close by Idle Timeout and I see error stacktraces in logs. (Actually it will happen only if my tests will be executed longer than 5 mins. Right now I don't have a lot of tests and the suit finishes before I see these stacktraces)

One should find a way to close the sessions after they're not longer required by the test case, so that they don't throw exceptions on timeout.
_Those exceptions are not relevant from a technical perspective but they may cause confusion of the user._