Guide: simplify testing section and move to end

[jkleint]

Reading the Testing section of the Guide, it's a bit long and involved. Partly because it introduces attributes, and partly because it's mostly error messages. It interrupts the pace and flow of the guide a bit with material that isn't really built upon later.

Perhaps we could trim it down to the basics of "Here's what a test looks like; here's where they go; here's how to run them?"

Testing is important certainly, but it is mostly conventions and tooling around the language, rather than a core language concept. For this reason, and because it isn't needed for later sections, could we profitably move it toward the end of the Guide, perhaps after section 24 or 25?

@steveklabnik, what say you?

[steveklabnik]

Testing is a very important concept, and deserves the full place in the guide. I actually already moved it back, the first draft it was 100% TDD 😉

it is mostly conventions and tooling around the language, rather than a core language concept.

The guide covers everything, including conventions and tooling. It's not just for the core language itself.