Add failing spec for codeblock sanitization

[greysteil]

Codeblock sanitization is hard. This PR adds another (currently failing) spec for it.

Connected to #1427.

[greysteil]

@pombeirp thanks for your work on #1427. Looks like it's broken the case where an inline code quote is closed implicitly by a line break. Any thoughts on a good way to fix?

[pedropombeiro]

@greysteil It looks like we can follow GitHub's rendering logic and avoid interpreting code fences which span a section separator (---) unless said code fence consists of 3 or more backticks. I'll see if I can look into it these next days (currently traveling back from vacation).

[pedropombeiro]

For context, these are the specs we should base the logic on (we should add these links to the sanitizer code for reference):

GFM code spans
GFM fenced code block

The current logic does a good enough job for code blocks (although it can be tightened up), but there are issues with the implementation for code spans. Namely, empty lines cannot appear inside a code span, however the current regex allows for it causing the unit test to fail.