[1.7.x] Fixed #22343 -- Disallowed select_for_update in autocommit mode

The ticket was originally about two failing tests, which are
fixed by putting their queries in transactions.

Thanks Tim Graham for the report, Aymeric Augustin for the fix,
and Simon Charette, Tim Graham & Loïc Bistuer for review.

Backport of b990df1d63 from master

tests/select_for_update/tests.py

@@ -134,6 +134,34 @@ class SelectForUpdateTests(TransactionTestCase):
             Person.objects.all().select_for_update(nowait=True)
         )
 
+    @skipUnlessDBFeature('has_select_for_update')
+    def test_for_update_requires_transaction(self):
+        """
+        Test that a TransactionManagementError is raised
+        when a select_for_update query is executed outside of a transaction.
+        """
+        transaction.set_autocommit(True)
+        try:
+            with self.assertRaises(transaction.TransactionManagementError):
+                list(Person.objects.all().select_for_update())
+        finally:
+            transaction.set_autocommit(True)
+
+    @skipUnlessDBFeature('has_select_for_update')
+    def test_for_update_requires_transaction_only_in_execution(self):
+        """
+        Test that no TransactionManagementError is raised
+        when select_for_update is invoked outside of a transaction -
+        only when the query is executed.
+        """
+        transaction.set_autocommit(True)
+        try:
+            people = Person.objects.all().select_for_update()
+            with self.assertRaises(transaction.TransactionManagementError):
+                list(people)
+        finally:
+            transaction.set_autocommit(True)
+
     def run_select_for_update(self, status, nowait=False):
         """
         Utility method that runs a SELECT FOR UPDATE against all