A month ago or so I was studying forth by writing a forth system... It's test suite is written in Test::Base that contains small programs and expected output.

To help me write the system I would first read a bit about the feature I was planning to add. Then I would start writing tests, and running them against an implementation like gforth, which I know isn't too drunk. By the time tests started accumilating, and were no longer failing I usually understood the feature better due to experimentation. Additionally I would have tests that check that the behavior of a given forth system is consistent with e.g. gforth, for that particular feature.

At that point I would switch the forth backend to use my implementation, with the missing feature, and implement things till they started working.

It was a very nice experience =)