Mox Lesson + Testing Lesson Updates

[SophieDeBenedetto]

This PR is opened in favor of this one from @fireproofsocks. It:

• Moves @fireproofsocks' new content on Mox to its own lesson under libraries/
• Update existing "mocking" section of Testing lesson to clarify and link to Mox lesson

[doomspork]

Two small styleguide change and this is good to go! Thank you @SophieDeBenedetto and @fireproofsocks

[doomspork]

@fireproofsocks do you have a twitter handle? I'd love to give you a shoutout when we announce this new lesson you've put together.

[fireproofsocks]

Thanks @doomspork ... although I have the @fireproofsocks handle on Twitter, I rarely use it. I also have https://medium.com/@fireproofsocks but most of the articles I write are paid contributions elsewhere, so there's not much to look at there, but I guess it wouldn't hurt to reference them. Thanks!