Documentation improvements (for testing)

Although [bug 2781](http://bugs.locamotion.org/show_bug.cgi?id=2781) is for the translate toolkit, there was an existing (although rather incomplete) section on testing in Pootle that included documentation on testing within TT as well as Pootle.  This PR updates those docs with information on additional py.test functions that we use and pulls over the existing docs from the (soon to be obsolete) wiki.

In theory, this could be duplicated or moved to the toolkit docs, but this seemed a bit simpler.