Test reconciler event handlers from informer

[zhongduo]

Problem
Currently we don't have an easy way to test event handlers of certain informers. This led to hard to notice problems like #1704. Similar problem is whether an informer resync will really happen, like in #1749. We should investigate in a possible testing framework to test similar problems that involves event handlers of informers. Our existing solution is to rely on e2e test, but it is difficult and time-consuming to test all these small corner case in e2e tests, unit test like test will provide more coverage.

Persona:
Which persona is this feature for?

Exit Criteria
Test framework that can easily cover #1704 and #1749

Time Estimate (optional):
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Additional context (optional)
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