Allow acceptance-testing transitions

[Leooo]

Not saying it's easy to add, but definitely missing:

To prevent transitions breaking over the lifecycle of an app, would it be thinkable to add a small test helper to be able to know which transitions are triggered when acceptance-testing a journey?

[ef4]

Having a test hook for logging which transitions run seems like a good idea.

In general, the testing supporting that's in ember-animated will be making its way into liquid-fire.