First pass at testing guide.

@bschwartz @MaxPower15 @jamesfzhang @JonathanTR @maryschmidt @mjuszczak @ryanartecona Would love your thoughts!
Hmm. I'm not super experienced on this, but my perception up until now has been that both have their strengths, and they aren't necessarily mutually exclusive. That being said, 100% agreed with your comment about avoiding a hodgepodge.

I don't have strong preferences towards either, but have found `let` to be much cleaner and more concise.