Hi Grzegorz,
I can reproduce the error using this sample project:
https://github.com/rh-desoftware/vaadin8_karaf_demo.
I assume you took the code for your itest from this repository.
I believe that WebSocket support is not active in your itest because you have only installed vaadin-push.
WebSockets for Vaadin 8 on Karaf 4.4.8 with Pax Web 8.0.33 is not working.
The repo contains an example for war and one for whiteboard. In both solutions the
"java.lang.IllegalStateException: Unable to configure jsr356 at that stage. ServerContainer is null"
occurs.