[1.5.x] Improved tests introduced in 04b00b6.

These tests are expected to fail for the file session backend because it
doesn't handle expiry properly. They didn't because of an error in the
test setup sequence.

Refs #19200, #18194.

Backport of 882c47c from master.

django/contrib/sessions/tests.py

@@ -277,9 +277,11 @@ class SessionTestsMixin(object):
         try:
             self.session['foo'] = 'bar'
             self.session.set_expiry(-timedelta(seconds=10))
-            self.session.create()
+            self.session.save()
+            old_session_key = self.session.session_key
             # With an expiry date in the past, the session expires instantly.
             new_session = self.backend(self.session.session_key)
+            new_session_key = new_session.session_key
             self.assertNotIn('foo', new_session)
         finally:
             self.session.delete(old_session_key)
@@ -353,15 +355,15 @@ class FileSessionTests(SessionTestsMixin, unittest.TestCase):
     backend = FileSession
 
     def setUp(self):
-        super(FileSessionTests, self).setUp()
         # Do file session tests in an isolated directory, and kill it after we're done.
         self.original_session_file_path = settings.SESSION_FILE_PATH
         self.temp_session_store = settings.SESSION_FILE_PATH = tempfile.mkdtemp()
+        super(FileSessionTests, self).setUp()
 
     def tearDown(self):
+        super(FileSessionTests, self).tearDown()
         settings.SESSION_FILE_PATH = self.original_session_file_path
         shutil.rmtree(self.temp_session_store)
-        super(FileSessionTests, self).tearDown()
 
     @override_settings(
         SESSION_FILE_PATH="/if/this/directory/exists/you/have/a/weird/computer")