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
machine.

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

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

tests/test-amend.t

@@ -485,14 +485,14 b' Modifying a file while the editor is ope'
   $ echo r0 > foo; hg commit -qAm "r0"
   $ echo alpha > foo; hg commit -qm "alpha"
   $ echo beta >> foo
-  $ cat > $TESTTMP/sleepy_editor.sh <<EOF
+  $ cat > $TESTTMP/touchy_editor.sh <<EOF
+  > sleep 1
+  > echo delta >> $TESTTMP/modify-during-amend/foo
+  > sleep 1
   > echo hi > "\$1"
-  > sleep 3
+  > sleep 1
   > EOF
-  $ HGEDITOR="sh $TESTTMP/sleepy_editor.sh" hg commit --amend &
-  $ sleep 1
-  $ echo delta >> foo
-  $ sleep 3
+  $ HGEDITOR="sh $TESTTMP/touchy_editor.sh" hg commit --amend
   $ if (hg diff -c . | grep 'delta' >/dev/null) || [ -n "$(hg status)" ]; then
   >   echo "OK."
   > else