Added assertion for the results of migrating an integer pk to SmallAutoField.

Follow up to 7ca42974ee which did the same for
similar tests.

tests/schema/tests.py

@@ -2160,6 +2160,19 @@ class SchemaTests(TransactionTestCase):
         with connection.schema_editor() as editor:
             editor.alter_field(SmallIntegerPK, old_field, new_field, strict=True)
 
+        # A model representing the updated model.
+        class IntegerPKToSmallAutoField(Model):
+            i = SmallAutoField(primary_key=True)
+
+            class Meta:
+                app_label = "schema"
+                apps = new_apps
+                db_table = SmallIntegerPK._meta.db_table
+
+        # An id (i) is generated by the database.
+        obj = IntegerPKToSmallAutoField.objects.create()
+        self.assertIsNotNone(obj.i)
+
     @isolate_apps("schema")
     @unittest.skipUnless(connection.vendor == "postgresql", "PostgreSQL specific")
     def test_alter_serial_auto_field_to_bigautofield(self):