tests: Log and save a diff if max_diff exceeds x86_64 tolerance
tests: Log and save a diff if max_diff exceeds x86_64 tolerance

This gives us a way to assess whether the diff is a real problem,
without breaking builds.

Signed-off-by: Simon McVittie <0e1aea601907c1a85c2dc96e5ea37e23c58d2e9b@debian.org>

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg