- Add in tvOS.  Update text for it also.
- Require a min version of the SessionFetcher with the needed support.

We've seen failures in tests when using xctool that we can't repro when using
xcodebuild, so even though xctool does a better job of managing the simulators
it doesn't seem to be safe to trust.

- Add a dispatch group to the ticket so the wait method can delay while
  callbacks and notifications are in flight.
- End the wait method's spinning when the ticket has been canceled rather than
  let it time out on cancel, since cancel suppresses the ticket's callback.
- Move the postNotificationOnMainThread utility method to the ticket since it's
  now done on the ticket's callback group.