Correct race condition in watch test
Correct race condition in watch test

It was possible for the randomized watch tests to `Stop` the watch
handles before they had the chance to deliver all of the enqueued
notifications, resulting in 1 fewer being delivered than should have
been. The test now properly waits for all notifications to come through
before ending the watches.

open-policy-agent/opa,open-policy-agent/opa,Eva-xiaohui-luo/opa,tsandall/opa,tsandall/opa,Eva-xiaohui-luo/opa,open-policy-agent/opa,tsandall/opa,open-policy-agent/opa,tsandall/opa,tsandall/opa,Eva-xiaohui-luo/opa,open-policy-agent/opa,open-policy-agent/opa,tsandall/opa