This isn't really related to #751 - it's a refactor of some of
our test logic, to avoid bloat of semi-repeated autofixture
customization attributes.
By switching to a behaviour-driven model, it's possible to
compose them, which means that there's no need to part-duplicate
between them. It should also reduce the overall number of them,
as combinations can be put together on-demand, rather than needing
a new attribute of their own.
