PageThumbs builds moz-page-thumb URLs with the "thumbnails" host and PageThumbProtocolHandler rejects
any other host, so the singular "thumbnail" spelling has not resolved since bug 1184701 changed it. The
context menu fixture's image therefore never loaded, and two of the protocol handler's error cases were
only re-testing host rejection rather than the query handling they describe.
Differential Revision: https://phabricator.services.mozilla.com/D318901