test-doctest: include dateutil
`mercurial.utils.dateutil` has dostrings that contain doctests. Include them.
Differential Revision: https://phab.mercurial-scm.org/D7519
--- a/tests/test-doctest.py Mon Nov 25 11:53:50 2019 -0800
+++ b/tests/test-doctest.py Mon Nov 25 12:33:06 2019 -0800
@@ -82,6 +82,7 @@
testmod('mercurial.url')
testmod('mercurial.util')
testmod('mercurial.util', testtarget='platform')
+testmod('mercurial.utils.dateutil')
testmod('mercurial.utils.stringutil')
testmod('hgext.convert.convcmd')
testmod('hgext.convert.cvsps')