Disable FOAF test as xmlns.com is down

[CasperWA]

It seems the xmlns.com site that hosts the FOAF ontology is down.
We should disable the test related to this until it is up again, possibly even include a list of available ontologies to test instead if one should be down. I.e., the idea would be to first try to retrieve FOAF, if it's down, the test tries another ontology, continuing in this way until a valid ontology is retrieved and tested, at which case the test ends.