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? :)
You suggested that last time, and I maintain my position that they are. :P  In theory, you could remove them and the sentence would be parseable, but it reads a lot more clearly with them.  The odd sentence construction is so that the (often large) haystack is the last thing in the error message.