Centralise fixtures under iati/tests/fixtures/

[hayfield]

At the moment test fixtures are spread around a number of different test files. This makes them hard to use for multiple purposes - for example, using a fixture that returns an XML string to test both the data and validator modules.

As part of #280, fixtures returning version numbers were centralised in a common location (iati/tests/fixtures/) that can be used across all the test files. This sort of approach should be used for fixtures of all types to make the code clearer, more powerful, and easier to maintain.