tests: make emacs test less strict
It was failing like this for me:
```
@@ -2,7 +2,7 @@
$ emacs -q -no-site-file -batch -l $TESTDIR/../contrib/hg-test-mode.el \
> -f ert-run-tests-batch-and-exit
Running 1 tests (*) (glob)
- passed 1/1 hg-test-mode--compilation-mode-support
+ passed 1/1 hg-test-mode--compilation-mode-support (0.045732 sec)
- Ran 1 tests, 1 results as expected (*) (glob)
+ Ran 1 tests, 1 results as expected, 0 unexpected (2020-09-11 16:49:56+0000, 0.046034 sec)
```
Differential Revision: https://phab.mercurial-scm.org/D9017
#require execbit
$ hg init repo
$ cd repo
$ echo foo > foo
$ chmod 644 foo
$ hg ci -qAm '644'
$ chmod 755 foo
$ hg ci -qAm '755'
reverting to rev 0
$ hg revert -a -r 0
reverting foo
$ hg st
M foo
$ hg diff --git
diff --git a/foo b/foo
old mode 100755
new mode 100644
$ cd ..