Put a timeout on page loading

[mthuurne]

In a run just now, the call to _URL_OPENER.open() in apetest.fetch.open_page() did not return. According to the backtrace, it was stuck on socket.recv_into().

There are probably other ways in which page fetching can get stuck, for example during the call to response.read(). It would be good to have a timeout and mark the page load as an error if the timeout expires.