`Listen.to` starts a bunch of background threads that need to perform
some work before they are able to receive events, but it doesn't block
until they are ready, which expose us to a race condition.

With `wait_for_state(:processing_events)` we can ensure that it's ready on
Linux, however on macOS, the Darwin backend has a second background thread
we can't wait on.

As a workaround we wait a bit after the fork to allow that thread to
reach it's listning state.