asyncnotifier tests: simplify the setup code

First change is to replace "range(len(self.NOTIFIERS)))" with
self.NOTIFIERS, as range(len(range(N))) ≡ range(N).

Sencond change is switch from an explicit indexing to use of zip.
Unittests still pass :)

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>