Add ember-raf-scheduler registerWaiter using `setupForTest` or document it? about ember-table

[acorncom]

@kpfefferle yeah, I saw your notes about your snags, which was part of the reason I wrote up the issue when I finally figured things out: "If Kevin is hitting this and he works at Addepar, how many other folks are also? ... 🤔" I'm delighted it helped you so quickly as well.

@mixonic any thoughts on preferences on how we tackle this?

[acorncom]

Fixed as part of #963 thanks folks!

[kpfefferle]

Soooo... this just changed my life. An app that I work on has been in the awful habit of pausing our tests for a split second to let this vertical-collection rendering finish before making assertions or interacting with the table. Thanks to this tip and the ember-raf-scheduler test waiter, we don't need to do this any more!

Given that even we didn't know that this would fix our issue, I definitely think we should at least add this to the documentation around testing. Anywhere else in the test helpers that we think it would be useful as an out-of-the-box solution would be great too IMO.

Thanks for bringing this to my attention, @acorncom!