brand short code light dark test for #12365

[gordonwoodhull]

Just adding a test and changelog for a shortcode api change.

#12365 was not a pure refactor; it did introduce an optional second parameter to the brand color shortcode

{{< brand color blue dark >}}

At shortcode time there is no user preference, so it defaults to light. ¹

I'll need to document this as well.

Footnotes

1. There could be author preference but I don't know if that's helpful — in an html context you will need to use both light and dark brand colors to create something that is mindful of both modes. This is an argument for making brandMode the first parameter, but I didn't want to break existing documents. ↩