The nascent XProc test suite is now on the web.
We're starting to collect tests for the XProc test suite. There are only a few at the moment, unit tests, and there's really no documentation for them, but you're welcome to take a peek.
Point your browser at the valid tests (that is, the tests that it is asserted are valid pipelines that produce unambiguous results) to start.
Each test summary includes the inputs expected on each port, the pipeline to run, and the expected outputs.
Note that some of the tests use pipeline documents as input. That's probably confusing, but I wrote most of these while testing my implementation so I did what was easy. We'll have better tests, with better navigation and more explanatory material, before we're done.