[Releng] Re-enable WebSocket tests after move to Jetty 10.0.12

Configure the Jetty servlet context handler to avoid IllegalException
during Net4JWebSocketServlet initialization leading to cmoent side
exception on each attempt to open a websocket.

Client side exception:

org.eclipse.net4j.util.lifecycle.LifecycleException:
 java.util.concurrent.ExecutionException:
  org.eclipse.jetty.websocket.api.exceptions.UpgradeException:
   org.eclipse.jetty.websocket.core.exception.UpgradeException:
     Failed to upgrade to websocket:
      Unexpected HTT  Response Status Code: 500 Server Error
	   at org.eclipse.net4j.util.lifecycle.Lifecycle.internalActivate

Change-Id: I7f942ee2dcff7e50d154760b84c19180a17e2dfa
Signed-off-by: Maxime Porhel <maxime.porhel@obeo.fr>
2 files changed