[1.9.x] Added a missing test method in tests/migrations/test_writer.py.

Backport of 5453aa66 from master

tests/migrations/test_writer.py

@@ -297,6 +297,8 @@ class WriterTests(SimpleTestCase):
             SettingsReference("someapp.model", "AUTH_USER_MODEL"),
             ("settings.AUTH_USER_MODEL", {"from django.conf import settings"})
         )
+
+    def test_serialize_iterators(self):
         self.assertSerializedResultEqual(
             ((x, x * x) for x in range(3)),
             ("((0, 0), (1, 1), (2, 4))", set())