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 <smcv@debian.org>
3 jobs for save-all-diffs in 7 minutes and 59 seconds (queued for 17 minutes and 9 seconds)
latest
Status Job ID Name Coverage
  Test
passed #131861
fedora test

00:07:37

 
  Lint
manual #131863
allowed to fail manual
clippy
passed #131862
rustfmt

00:00:22