Adding spin test documentation

[tpmccallum]

Content must go through a pre-merge checklist.

Pre-Merge Content Checklist

This documentation has been checked to ensure that:

• [x] The title, template, and date are all set
• [x] Does this PR have a new menu item (anywhere in templates/*.hbs files) that points to a document .md that is set to publish in the future? If so please only publish the .md and .hbs changes in real-time (otherwise there will be a menu item pointing to a .md file that does not exist)
• [x] File does not use CRLF, but uses plain LF (hint: use cat -ve <filename> | grep '^M' | wc -l and expect 0 as a result)
• [x] Has passed bart check

$ bart check content/spin/v2/testing-apps.md 
✅ content/spin/v2/testing-apps.md

• [x] Has been manually tested by running in Spin/Bartholomew (hint: use PREVIEW_MODE=1 and run npm run styles to update styling)

• [x] Headings are using Title Case
• [x] Code blocks have the programming language set to properly highlight syntax and the proper copy directive
• [x] Have tested with npm run test and resolved all errors
• [x] Relates to an existing (potentially outdated) blog article? If so please add URL in blog to point to this content.

[rylev]

This seems to be the post that I wrote. I had originally meant that as a blog post and not as documentation. I'm more than happy to let this live as documentation (since that's generally longer lived anyway), but then I'm not quite sure if we want to also have a blog post, and if so what that blog post should say.