Contributing to documentation - Topic/Testing

Restructured a lot of this content from the documentation after I was sifting through it today in an attempt to get started with testing in SilverStripe... It was a bit all over the place - hopefully this will be a bit less daunting to someone to go through than it was for me.

When I have time (and after I have figured out more on the testing side of things in SS), I will add some more changes. I'd like to re-write the functional tests page, though I don't know enough to do that comfortably as of yet. 

Here are my changes for now anyway.