Change bug878194 to an |eq| test

While working on simplifying the approach in PR #5383, I noticed that in some cases it is possible (caused by incorrect code) to truncate an inline image stream in such a way that it still represents a valid image.
To make sure that PR #5383 doesn't cause any regressions, I thus think that it would be good to change `bug878194` to an `eq` test instead.