M2 Testing tdd mocha chai lesson updates

[hfaerber]

3 PRs on 3 separate repos need review for issue #608

• This PR for the M2 Testing: Tdd, Mocha and Chai lesson
• PR for updates to practice repo within M2 Testing lesson
• PR for updates to practice repo in M3 Testing With Hooks Lesson

Changes Made in this PR

• M2 Testing: Tdd, Mocha and Chai lesson:
  • Replaced class examples with function examples. Some are pure, some have side effects.
    • Decided to included functions with side effects because we are likely to see this in M2 code. We want to be able to show them how to still test those side effects. This also opens the door to having students think about how purity and side effects can impact whether a function is easier, harder or impossible to test.
• M2 Testing With Hooks lesson
  • Replaced references to classes with functions

Requested updates from ticket that were not implemented:

• As the students are heavily focused on iterator methods at this point, one or more of the tests in the examples should be pushing them to write a function that uses an iterator.
  • Reason I didn't implement this bullet in my initial changes:
    • The testing lesson is taught before iterators are introduced.
    • I'm concerned that adding a more complex example with an iterator would take time away from the hands-on test writing activity at the end of the lesson which is where the real learning is happening.
    • This is their only lesson about writing tests. Adding an unfamiliar example could pull focus away from the learning goals.

Questions for Reviewer:

• Should I add an examples to include an iterator? See notes above

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[niksseif]

Hey Heather! Thanks for working on this. I haven't had the chance to review this pull request entirely yet, but I noticed that Netlify is not passing one of your changes. You may want to update the image address from "http://i.imgur.com/T7Q4YkE.png" to "https://i.imgur.com/T7Q4YkE.png".

[hfaerber]

Fixed errors based on Cass's comments. Merging to development.