Adds testing documentation

[cannikin]

I'm holding up this Rails Guide to testing as my gold standard: https://guides.rubyonrails.org/testing.html

It describes everything that Rails provides for testing, and how to test all the various parts you would build (models, controllers, jobs). It's crazy thorough and is usually the first place I go if I forget how to write a test for something, even before Google. And most Google queries will return this doc as the first result anyway.

[cannikin]

Trying out some inline code execution with JSFiddle: https://deploy-preview-530--redwoodjs.netlify.app/docs/testing#a-simple-test

[thedavidprice]

Nice!

Not sure you need more options, but Docusaurus v2 uses React Live for .mdx:
https://v2.docusaurus.io/docs/markdown-features/#interactive-code-editor

[jtoar]

@cannikin This is amazing! 🤯 Planning to publish a review on this tomorrow, just takes time to enter changes from my editor into GitHub's code review. Looking at a few vscode extensions...

For now, consider this my only suggestion.

[cannikin]

Thanks! It's only probably half-way done and I haven't done a full run-through to fix flow issues, or grammar and typos. I'd love a review but you might want to wait until after all that so I get a chance to catch all the embarrassing mistakes. :)

[jtoar]

@cannikin Sure thing--just let me know

[cannikin]

Oh geez, almost up to 6,000 words and I haven't event got into server-side testing yet. 😬

[cannikin]

First draft done, 8,000 words. 😬 Let me take a pass through and fix anything glaring and then I'll hit you up @jtoar, probably tomorrow!

[cannikin]

Okay @jtoar I think it's ready! Grand total: 8,552 words (including code, however wc decides to count that).

I did a big refactor on the section headings to get the right nav much easier to browse to a topic.

[jtoar]

@cannikin I think I could make more edits, particularly to the latter half of the document just because I started at the top and got less and less keen as I was going, but this is a good start, and don't want to hold off reviews any longer. Let me know what you think; a good amount are just "capitalize this" because I feel like @thedavidprice and I have gone back and forth about capitalizing "Redwood nouns" like Cell, Service, etc. I think it's helpful to do so!