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