When `@require`ing local files (with the `file://` scheme) the
greasemonkey manager was not catching the DownloadItem.finished signal
because it was being emitted before it had managed to connect.
I didn't see this happening while testing with files that should have
been in cache but I wouldn't be surprised.
I had to change the download mock to be able to give it the appearance
of asynchronicity. Now when using it one must set download.successful
appropriately before firing download.finished. I also added a list of
downloads to the stub so a test could enumerate them in case the
unit-under-test didn't have a reference to them.
7dab8335e2