Update message_channel.js for closed ports
Don't automatically start the test when the sandbox is started

This should fix the NETWORK_ERR: XmlHttpRequest Exception 101

It seems that terminating a webworker while it's still loading
scripts through `importScripts` triggers this exception.

The goal is then is to start the test only once the sandbox is fully
initialized.
Allow multiple sandboxes in the same environment.

Event handlers need to check that the source of the event is the
targeted iframe.

When accessing the `contentWindow` attribute of an iframe not inserted
in the DOM, IE8 generates an exception (other browsers return null)