I'm not convinced this adds any value beyond the existing success case

StorageServer hides all of the networking details in the real implementation.
If tahoe-great-black-swamp works then the existing success case should
exercise all of the same logic in gbs-downloader whether we're actually using
it or not...

It might be nice to move all of the StorageServer implementations into
gbs-downloader and then test that they really do behave the same as each
other. Right now they're mostly in tahoe-chk though so that's annoying.