Run tests against all documents in Pandoc repo

[zspitz]

https://groups.google.com/g/pandoc-discuss/c/gQRR9qat9Zw

Download all the documents in https://github.com/jgm/pandoc/tree/master/test; write a separate project to do this.

For each document:

• Pass the document into Pandoc and return the AST
• Pass the document into Pandoc using test filters for raw and high-level, and return the post-filter AST
• Assert the two ASTs are the same

[zspitz]

The test matrix will be as follows:

• Raw / high-level
• Standard visitor / delegate visitor
• Windows / Linux (help needed)

Running the tests on AppVeyor will require installing Pandoc first. This should be straightforward using an AppVeyor Windows image, there is a Choclatey Pandoc package available; but I've had some trouble trying to use the Pandoc container with GitHub Actions in the past.

[zspitz]

This is complete, save for #12. Closing.