tests: add unit tests for Markdown

Lars Wirzeniusrequested to merge
liw/mdtests into main

Description

A necessary step before we rewrite the markdown parsing to use pulldown_cmark. Adds some unit tests.

Checklist

  • You have read, and followed the guidance in, the CONTRIBUTING guide
  • The MR content has run through a pipeline, or at least has pipelines enabled.
  • There are no licence concerns with this contribution (The README clearly states Subplot is MIT-0)

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