test(cheddar): Add simple Markdown rendering tests

Covers some of the odd things we do, specifically callouts and code
rendering.

Change-Id: Ib8542373b434b53d277b0d8c9ddb78ac7c5176a5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2689
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tools/cheddar/src/main.rs

@@ -25,6 +25,9 @@ use syntect::html::{
     append_highlighted_html_for_styled_line, start_highlighted_html_snippet, IncludeBackground,
 };
 
+#[cfg(test)]
+mod tests;
+
 lazy_static! {
     // Load syntaxes & themes lazily. Initialisation might not be
     // required in the case of Markdown rendering (if there's no code