Add IntegrationTestCase

One of our projects contains an `IntegrationTestCase`, which extends `RequestTestCase` with a call to `render()` that puts the HTML response into a `string`.  This is generally useful, and I think it should be added here.

@meshy second opinion? :)
Putting it in parentheses also works syntactically, but de-emphasises the important part of the message (i.e. the actual error).  Putting it between `count` and `instances` makes it sound even more archaic (and needing of commas) :P