Investigate flaky test

[askpt]

Not certain why, it looks like I potentially introduced a flaky test in #462. I will take a look later today and see if I can reproduce it

Originally posted by @kylejuliandev in #415 (comment)

We have a flaky test: VerifyMultipleHandlersAreRegisteredAsync. We need to investigate why multiple handlers are registered (4, 3 vs 2.).

[kylejuliandev]

I suspect Api.Instance state is being intermittingly shared between test cases. I think this explains why sometimes we see 2, 3, or 4 handlers. 4 handlers are added in total between the three unit tests for the AddHandler code.