Fixed #28478 -- Make DiscoverRunner skip creating unused test databases.

SimpleTestCase.databases makes it possible to determine the set of
databases required to run the discovered tests.

tests/test_runner_apps/databases/tests.py

@@ -0,0 +1,18 @@
+import unittest
+
+
+class NoDatabaseTests(unittest.TestCase):
+    def test_nothing(self):
+        pass
+
+
+class DefaultDatabaseTests(NoDatabaseTests):
+    databases = {'default'}
+
+
+class OtherDatabaseTests(NoDatabaseTests):
+    databases = {'other'}
+
+
+class AllDatabasesTests(NoDatabaseTests):
+    databases = '__all__'