Add canonical copy of structure fixtures to this repository

[alerque]

This repository already has one set of reference fixtures that are used by the Rust, Python, and Javascript implementations. I'm working on adding Lua to that list. Unfortunately these fixtures are not great for development because they are pretty much all-or-nothing.

I see that two implementations, Python and JavaScript have another set of test fixtures that are much more useful for development purposes because they are more focused on testing one syntax issue at a time. It isn't obvious from the repositories which project got these first (probably JS, then copied to Python) or where changes to them would be made.

I suggest adding that set of fixtures to this repository as a canonical set of tests. This would make it much more clear what downstream projects could/should copy.

[Pike]

I'd prefer to get rid of the structure tests, actually. That's projectfluent/fluent.js#305, I guess.

As for the reference tests, if they're structured in an unhelpful way, that'd be a good reason to refactor them. Resummarizing to that.