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? :)
Yeah, I'm [with Ian](https://github.com/incuna/incuna-test-utils/pull/18#discussion_r16981703) on that one -- I believe that the last comma makes the sentence harder to parse.

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