First pass at testing guide.

@bschwartz @MaxPower15 @jamesfzhang @JonathanTR @maryschmidt @mjuszczak @ryanartecona Would love your thoughts!
I think there's value in having a convention; otherwise different people use different syntaxes and we wind up with a hodgepodge of patterns and have to keep track of twice as many things when reading code.

I often find `let` blocks harder to read than `before` blocks because they create indirection and run things out of order. The procedural way of doing it is to use more `describe` blocks with `before` and `after` blocks inside them so that you are only defining and running things in the places where they are needed and used.

This is purely an issue of preference. `before`/`after` is more clear but more verbose; `let`is less clear but more concise. I favor the former but could be persuaded toward the latter. My only strong preference is to settle on one convention where possible.