Add Manuel testing

This adds 54 tests based on the documentation for test authors. These tests have been written by taking each of the existing code snippets and wrapping them in a TestCase.

We then collate these in to a test suite at the bottom of the document and run it, as Manuel (unsurprisingly) doesn't support embedding TestCases within documentation.

This addresses https://bugs.launchpad.net/testtools/+bug/1183332 (at least partially).