Add tests for AbbreviatingSerializer and AbbreviatingParser roles

[kasei]

Test end-to-end abbreviation support for:

• SPARQL
• Turtle (depends on Add AtteanX::Serializer::Turtle serializer #52)
• RDF/XML (depends on Port RDF::Trine::Serializer::RDFXML to Attean #22)

as well as cross-format tests for Turtle and RDF/XML.

For example, tests should:

• Ensure all @Prefix declarations remain intact in turtle-to-turtle transcoding
• 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
• Ensure SPARQL and Turtle uses PrefixNames for all IRIs that can be abbreviated

[kasei]

Code for SPARQL and Turtle is now done. I'm closing this and opening a new ticket (#91) for just the RDF/XML work, as that depends on #22 that doesn't yet have a schedule for completion.