`Dispatcher` internal await is uncancelable in 3.5.x

[djspiewak]

Relevant wayfinder:

              if (regs.isEmpty) {
                // TODO this needs to be modified to be cancelable
                F.async_[Unit] { cb =>
                  if (!latch.compareAndSet(Noop, () => cb(Completed))) {
                    // state was changed between when we last set the latch and now; complete the callback immediately
                    cb(Completed)
                  }
                }

This code predates the async cancelability change and I'm relatively certain that it is intended to be cancelable. We don't have any tests which exercise that codepath though, so this qualifies as an invisibug.

[djspiewak]

Marking this as a good first issue since it's a really trivial fix. The hard part is writing a test case to reproduce it, but that shouldn't be too terrible with ticked/TestControl.