Streamlined quickcheck tests by removing duplication

All tests are now defined in Data.RDF.GraphTestUtils.hs . New graph
implementations only need to provide implementations of

empty'     :: RDF rdf => rdf
mkRdf'     :: RDF rdf => Triples -> Maybe BaseUrl -> PrefixMappings -> rdf
triplesOf' :: RDF rdf => rdf -> Triples

See Data.RDF.MGraph_Test.hs for an example. Add your graph
implementation to `defaultMain` in Test.hs .
Merged cordawyn/master