Don't import images in tests

[rszsz]

While it makes sense in benchmarks, this is probably not a very good approach:
https://github.com/naisuuuu/mangaconv/blob/59a96e853a01a2d8df601723f84a2da276ead893/imgutil/grayscale_test.go#L90-L93
We should inline sufficiently complex input and output images using their appropriate *image.Types.
This is more of a nice-to-have than a functional requirement, so I'm leaving this issue as a reminder for the future.