Final call: Add test case design instructions

Hey folks,
This pull request adds instructions on how to write good test cases. On
what test cases to include, and what to omit. This is a substantial
addition to our process so we're giving this one a two week final call. If
you have any feedback, please leave a review before August 28th.

https://github.com/act-rules/act-rules.github.io/pull/1356

-- 
*Wilco Fiers*
Axe for Web product owner - Co-facilitator WCAG-ACT - Chair ACT-R