Simplified temporary directory handling in AdminScriptTestCase.

Use tempfile.TemporaryDirectory() in AdminScriptTestCase.setUp()
to create and destroy a temporary directory for each test. It removes
the need for individual tests to delete files.

For test classes that don't use the temporary directory, inherit from
SimpleTestCase.

tests/urlpatterns_reverse/tests.py

@@ -534,14 +534,12 @@ class ReverseLazySettingsTest(AdminScriptTestCase):
     import error.
     """
     def setUp(self):
+        super().setUp()
         self.write_settings(
             'settings.py',
             extra="from django.urls import reverse_lazy\nLOGIN_URL = reverse_lazy('login')",
         )
 
-    def tearDown(self):
-        self.remove_settings('settings.py')
-
     def test_lazy_in_settings(self):
         out, err = self.run_manage(['check'])
         self.assertNoOutput(err)