Re-organize PhpStorm testing docs

Move them into the dedicated page on PhpStorm. Why? Tim has expressed
some concern about the potential difficulty (and slippery slope) of
starting to maintain IDE-specific docs. I like the PhpStorm page, but I
can see where he's coming from to a certain extent. As such, I think it
will be a good compromise if we can keep all the IDE-specific stuff as
consolidated as possible.