* Test that downloading the HTML page downloads the full size image
instead.
* Test that downloading a small or medium size image downloads the
full size instead.
* Test the above for both single illustrations and for manga galleries,
for new and old posts (i.e. uploaded before the Pixiv URL changes versus
after), and for different file extensions.
* Test trying to download ugoira zip files. These tests are expected to
fail because we don't support ugoira yet.
* Also add some more tests for fetching source data.
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