Add integration test for ExtendedURLField

tests/integration/forms/field_tests.py

+from oscar.forms import fields
+from django.test import TestCase
+
+
+class TestExtendedURLField(TestCase):
+    """ExtendedURLField"""
+
+    def test_validates_local_urls(self):
+        field = fields.ExtendedURLField(verify_exists=True)
+        field.clean('/')