Fixed #20552 -- Corrected release notes reference to name of new test discovery runner.

``models.py`` and ``tests.py`` modules of a Python package in

:setting:`INSTALLED_APPS`.

The new runner (``django.test.runner.DjangoTestDiscoverRunner``) uses the test

discovery features built into unittest2 (the version of unittest in the Python

2.7+ standard library, and bundled with Django). With test discovery, tests can

be located in any module whose name matches the pattern ``test*.py``.

The new runner (``django.test.runner.DiscoverRunner``) uses the test discovery

features built into unittest2 (the version of unittest in the Python 2.7+

standard library, and bundled with Django). With test discovery, tests can be

located in any module whose name matches the pattern ``test*.py``.

In addition, the test labels provided to ``./manage.py test`` to nominate

specific tests to run must now be full Python dotted paths (or directory