chore: Serializer Spec tests

[jeswr]

Just realized that serializer spec tests appear to be missing on this repository - it is also unclear to me if rdf-test-suite currently supports spec tests for anything other than JSON-LD (@rubensworks ?).

[rubensworks]

Are there any serializer tests for other syntaxes besides JSON-LD?
In any case, rdf-test-suite does not handle these atm indeed.

[jeswr]

The behavior I was expecting is that rdf-test-suite would use the existing syntax and semantics tests to perform serialization tests, by checking that the result of parsing the output of the serializer is isomorphic to the original syntax test document.

I can raise an issue on rdf-test-suite and do some work towards this if this is a desirable addition. I'd find it useful for testing https://github.com/jeswr/pretty-turtle

[rubensworks]

The JSON-LD serialization tests work in a slightly different way.
There we start with RDF, and serialize to JSON(-LD), and then match JSON with the expected output.
In Turtle and such, this would be a bit more difficult, as you'd need a way to account for empty spaces and other choices the parser can make.

In any case, your approach could work as well, but since the tests are not designed for this, I wouldn't make this standard behaviour. Maybe behind a flag or so.