Stop printing diffs between failed image matches

[Jacalz]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

When image tests are failing, the assert.Equal call prints a massive stream of both all of the uint8{} pixel values but also a diff between the images which results in a total flooding of the terminal. I find the output to be not all that helpful but also distracting as it is hard to see which tests are failing.

Is it possible to construct a solution with the existing API?

Should be possible to just stop using assert.Equal for anything else than SVG but I had problems doing this change due to weird issues related to #4976.

Describe the solution you'd like to see.

No diff for failing images matches when the image is anything different than an SVG.