test: definition, heading, and list together

[ChristianMurphy]

breaks expectation at https://github.com/wooorm/markdown-rs/blob/b910e37bb387a62509a7fda26617e9870ed6d56f/src/to_html.rs#L197

[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