Introduce mockito for mocking in tests

[robintw]

I recently came across the mockito library for creating mocks as part of tests. We use quite a lot of mocks, and the way we create them (using the built-in Python libraries for doing so) can be fairly confusing at times, and are potentially rather fragile. This library looks like a far nicer way to create mocks, and so we should consider starting to use this - and potentially migrating some of our old tests to use this.

There is a good walkthrough on how the library works.

[IanMayo]

Yes, it looks good, and may reduce the volume of test code we need to create/maintain.