Extract test helper method wait_for_rails_to_boot

[andyw8]

With #526 we'll have this in 3 places, so let's extract it to something more readable.

(fyi @ChallaHalla)

[vinistock]

Instead of sleeping, can we instead extract this one to a shared method and reuse it everywhere?

It's a bit more elegant because it forces the threads to block until activation is complete.