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? :)
I've just realised I've been misreading the sentence. Maybe `but found {actual_count}` should be in parentheses instead of commas?