Add RDF/XML tests for AbbreviatingSerializer and AbbreviatingParser roles

[kasei]

Test end-to-end abbreviation support for RDF/XML and cross-format tests for Turtle and RDF/XML (depends on #22).

For example, tests should:

Ensure all xmlns declarations remain intact in rdf/xml-to-rdf/xml transcoding
Ensure all xmlns declarations remain intact in rdf/xml-to-turtle transcoding
Ensure all used @Prefix declarations remain intact in turtle-to-rdf/xml transcoding
Ensure RDF/XML uses PrefixNames for predicate IRIs that can be abbreviated

(This was split from #53.)