Ignore namespace prefix tests broken by default JSONLDMode.EXPAND

Until there is a way to override the WriterConfig for RDFWriterTest,
need to ignore tests that fail because RDFFormat.JSONLD declares that it
supports namespaces, but the default mode, EXPAND, does not preserve
them. To preserve namespaces, users must setup WriterConfig with
JSONLDMode.COMPACT, as demonstrated in this commit