Document the refactoring test language

[jackfirth]

It's incredibly difficult to test refactoring rules without #lang resyntax/testing/refactoring-test. The testing language ought to be documented and exposed to users, so custom rules created by users can be tested. This will also make it easier for other people to contribute to Resyntax.

The language could probably do with a snappier name too. Perhaps just #lang resyntax/test.