Documentation missing for template tests

[alanpearce]

I thought nunjucks didn't have support for Jinja-style tests until I found the PR that added it (#1033). I wanted to add my own tests Environment.addTest does not appear in https://mozilla.github.io/nunjucks/api.html. They don't appear very clearly on https://mozilla.github.io/nunjucks/templating.html either; only select and reject hint that they exist without listing the built-in tests

Environment.addTest and friends are also missing in https://github.com/DefinitelyTyped/DefinitelyTyped, but I don't know if anyone from here has an eye on the TypeScript types.