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
Differential Revision: https://phab.mercurial-scm.org/D8065