WIP: Initial warning about cross-process marshalling: Issue #996

[craigfowler]

This catches me out every darn time! I feel like it should be more obvious and pointed out. In this case I've added an object as a parameter which is attached to an event bus which is maintained outside of the test execution. This means that the event bus has an affinity with the "test discovery & creation" process wheras the things submitting events to it are in the "test execution" process. Of course, this results in two 'copies' of the event bus, and any listeners to it (which were attached from the "discovery & creation" process) don't actually receive any of the events sent by the test execution.

I'm not 100% sure this is factually correct yet; I could use some assistance from maintainers to be confident that it's right. At the moment this PR is more of a reminder to myself to look further into it and improve upon it.

[craigfowler]

I've opened #996 to describe the initial problem I'm trying to solve. There are probably other places where this could be signposted-from, such as the docs for those interfaces/methods which are executed "in test suite/case discovery/creation"-land.

I could use some help from maintainers to point that out, as well as to fact-check that I'm even correct (based on a half-recalled explanation from long ago).