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/test_runner/tests.py

@@ -155,14 +155,12 @@ class CustomTestRunnerOptionsSettingsTests(AdminScriptTestCase):
     through a settings file.
     """
     def setUp(self):
+        super().setUp()
         settings = {
             'TEST_RUNNER': '\'test_runner.runner.CustomOptionsTestRunner\'',
         }
         self.write_settings('settings.py', sdict=settings)
 
-    def tearDown(self):
-        self.remove_settings('settings.py')
-
     def test_default_options(self):
         args = ['test', '--settings=test_project.settings']
         out, err = self.run_django_admin(args)
@@ -195,11 +193,9 @@ class CustomTestRunnerOptionsCmdlineTests(AdminScriptTestCase):
     using --testrunner.
     """
     def setUp(self):
+        super().setUp()
         self.write_settings('settings.py')
 
-    def tearDown(self):
-        self.remove_settings('settings.py')
-
     def test_testrunner_option(self):
         args = [
             'test', '--testrunner', 'test_runner.runner.CustomOptionsTestRunner',
@@ -228,11 +224,9 @@ class CustomTestRunnerOptionsCmdlineTests(AdminScriptTestCase):
 
 class Ticket17477RegressionTests(AdminScriptTestCase):
     def setUp(self):
+        super().setUp()
         self.write_settings('settings.py')
 
-    def tearDown(self):
-        self.remove_settings('settings.py')
-
     def test_ticket_17477(self):
         """'manage.py help test' works after r16352."""
         args = ['help', 'test']