Make browser testing more resilient by checking test loading

[whesse]

Our hardest failures to find are those where a browser test times out because it doesn't load.
The sequence of actions that should happen is:
Driver page requests a test URL from the controller
Driver page fetches that URL from the file server
The page loads, in an iframe or window
The page reports that it started (entered main).

The driver page, test controller, and file server could communicate more, so that when one of these steps doesn't happen, we detect it and report details about the problem. If the driver page writes a URL to the iframe's address, but it doesn't get the started message, it should return fuller data about the browser state, or perhaps close and reopen the iframe.