React Testing Library Course updates

[helloitsjoe]

Making a few updates to the React Testing Library course based on some feedback/conversations we've had.

1. Removing Session 1 (testing components without a library)
2. Making a few of the tests a little more true-to-life
3. Updating some of the wording
4. Removing the setup code

   const mock = jest.fn()

   and

   const { getBy...} = render(<Comp />)

   Let me know what you think about this!