Add integration test for RDF output

[JeroenDeDauw]

AC

• Verifies that RDF output for an item contains its mappings
• Verifies that the RDF output for an item without mappings does not contain mappings

[JeroenDeDauw]

The purpouse of this test is to verify that the service wiring and hooking into WIkibase is functioning. It is not about verifying details in the RDF output. So some simple assertStringContains on the entire RDF is sufficient.