test: definition, heading, and list together

[ChristianMurphy]

breaks expectation at

markdown-rs/src/to_html.rs

Line 197 in b910e37

let last_buf = last_buf_opt.expect("at least one buffer should exist");

[wooorm]

This is about how definitions, setext headings, and lists interplay.
Here’s a reduced test case:

"[a]: b\n-\n-"

[ChristianMurphy]

Also found a way to replicate the issue with only setext headings (and list like content).

a
-
--
a
-

which could potentially be related to #31, but shows up in a different way