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_discovery_sample/pattern_tests.py

@@ -0,0 +1,7 @@
+from unittest import TestCase
+
+
+class Test(TestCase):
+
+    def test_sample(self):
+        self.assertEqual(1, 1)

tests/test_discovery_sample/tests/tests.py

@@ -0,0 +1,7 @@
+from unittest import TestCase
+
+
+class Test(TestCase):
+
+    def test_sample(self):
+        pass

tests/test_discovery_sample/tests_sample.py

@@ -0,0 +1,22 @@
+from unittest import TestCase as UnitTestCase
+
+from django.test import TestCase as DjangoTestCase
+from django.utils.unittest import TestCase as UT2TestCase
+
+
+class TestVanillaUnittest(UnitTestCase):
+
+    def test_sample(self):
+        self.assertEqual(1, 1)
+
+
+class TestUnittest2(UT2TestCase):
+
+    def test_sample(self):
+        self.assertEqual(1, 1)
+
+
+class TestDjangoTestCase(DjangoTestCase):
+
+    def test_sample(self):
+        self.assertEqual(1, 1)