TestSingleFileEventHandler: abstract notifier type

Rather than hardcode that we have two notifiers, and notifier 0 is the
terminating one, we abstract this with class level constants. This makes
it easier to add more, with different features.

The only real change is that now the callback class takes as input the
whole test object, rather than just the notified array, to have access
to those constants.

The rest is just replacing of hardcoded 0s and 1s with
self.NOTIFIER_TERM and self.NOTIFIER_NORM, and of notifier_count with
len(self.NOTIFIERS).

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>