Fixed #17365, #17366, #18727 -- Switched to discovery test runner.

Thanks to Preston Timmons for the bulk of the work on the patch, especially
updating Django's own test suite to comply with the requirements of the new
runner. Thanks also to Jannis Leidel and Mahdi Yusuf for earlier work on the
patch and the discovery runner.

Refs #11077, #17032, and #18670.

tests/test_runner_deprecation_app/tests.py

@@ -0,0 +1,9 @@
+import warnings
+
+from django.test import TestCase
+
+warnings.warn("module-level warning from deprecation_app", DeprecationWarning)
+
+class DummyTest(TestCase):
+    def test_warn(self):
+        warnings.warn("warning from test", DeprecationWarning)