Use regex comparison in WebSocket stats test

The output includes thread pool information which appears to not vary
enough to cause failures locally but does so on the build server.
Re-obtain port on every WebSocket integration test

This change adds a WebSocketTestServer setup method that initializes
the server and obtains a new port.

In turn AbstractWebSocketIntegrationTests invokes the new method from
its setup method thus ensuring a new port is used on every test.