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)