test: simplify test-amend.t to avoid race condition

Insted on relying on sleep, we could simply have the editor do the file change.
This remove the reliance on "sleep" and avoid test failing on heavy load

To test this, I reverted the code change in 5558e3437872 and the test started
failing again.

This is a graft on stable of 141ceec06b55 which should have targeted for stable.
12 jobs for topic/stable/flacky-test in 50 minutes and 47 seconds (queued for 30 minutes and 44 seconds)