Add compatibility tests

[ehuss]

I think it would be good to have some tests that validate the preprocessor and renderer interfaces remain compatible with plugins built with older versions. That could help catch regressions like #1571 and others.

[earv1]

Hey there, I'm interested in your project as a first time contributor. I started looking at this, but it seems like this might already be solved by #1897. Is there anything still missing?

[ehuss]

That doesn't quite do what I was thinking about for this issue. That has a hard-coded input that applies to the current preprocessor. The intent here is that the current mdbook's output can be consumed and processed by an old preprocessor (and renderer).

I'm not sure how the test could be best structured, but essentially I was thinking that there would be a preprocessor and renderer that just assert that their input matches a hard-coded value. The test would run mdbook against that preprocessor/renderer combination, and if either fail, then the test fails. That would help detect if mdbook starts generating different structures.

I'm not sure how maintainable that is, I haven't thought about it too much.