Add "Testing Day Checklist" to the Github wiki

[jaclinec]

In the course of carrying out usability testing for components in Q3 2023, we created a "Testing Day Checklist" to serve as a tool to help streamline routine testing day procedures. We decided it'd be helpful to offer a version of this checklist to the USWDS community. We decided to publish this checklist in the uswds Github Wiki,

[jaclinec]

The Usability Testing Day Checklist wiki page is now published, but I'd like for Content Strategy (@sarah-sch , @katieekline )to take a look at the drafted content and add comments/edits. I can update the wiki page based on their recommendations.

[brunerae]

Closing as complete.