`EventHandlerHelper` is a bit of a strange beast. It's really a specialised collection of helpers (like `ApplicationHelper` and friends in #783), but it's used in the tests for `go.vumitools.subscription` as well and therefore can't really live in `go.apps.tests.helpers`.

The magic setup stuff is all the common boilerplate extracted from the various event handler tests that use it because it's mostly irrelevant to the thing being tested but still needs to be there for stuff to work.