Test refactoring work; trade out wru for nodeunit.

This is a work in progress; `tests/validate.js` is working, and I'm handing off
the remaining test harnesses to @gareth for conversion.
Automatically re-run postinstall when running tests.

This may not be a sound strategy long-term, but I think (at least for now) it'll
help people make sure their compiled fixtures are up-to-date.