Stylistic cleanup of Postgres autocommit tests

Cleaned up tests introduced in f572ee0.

Thanks to Claude Paroz for suggesting the changes.

tests/regressiontests/transactions_regress/tests.py

@@ -175,6 +175,8 @@ def test_failing_query_transaction_closed_debug(self):
         self.test_failing_query_transaction_closed()
 
 
+@skipUnless(connection.vendor == 'postgresql',
+            "This test only valid for PostgreSQL")
 class TestPostgresAutocommit(TransactionTestCase):
     """
     Tests to make sure psycopg2's autocommit mode is restored after entering
@@ -196,6 +198,9 @@ def setUp(self):
         new_backend = self._old_backend.__class__(settings, DEFAULT_DB_ALIAS)
         connections[DEFAULT_DB_ALIAS] = new_backend
 
+    def tearDown(self):
+        connections[DEFAULT_DB_ALIAS] = self._old_backend
+
     def test_initial_autocommit_state(self):
         self.assertTrue(connection.features.uses_autocommit)
         self.assertEqual(connection.isolation_level, self._autocommit)
@@ -222,14 +227,6 @@ def test_transaction_stacking(self):
         transaction.leave_transaction_management()
         self.assertEqual(connection.isolation_level, self._autocommit)
 
-    def tearDown(self):
-        connections[DEFAULT_DB_ALIAS] = self._old_backend
-
-TestPostgresAutocommit = skipUnless(connection.vendor == 'postgresql',
-    "This test only valid for PostgreSQL")(TestPostgresAutocommit)
-TestPostgresAutoCommit = skipUnlessDBFeature('supports_transactions')(
-    TestPostgresAutocommit)
-
 
 class TestManyToManyAddTransaction(TransactionTestCase):
     def test_manyrelated_add_commit(self):