Consider use of t.Cleanup in goleak

[prashantv]

Within a test, the recommendation is to use defer goleak.VerifyNone(t). However, this can't be used inside of a test helper. We may want to support t.Cleanup, possibly via a new method, so this issue is to investigate what we should do.

[Manik2708]

How about wrapping the VerifyNone inside t.CleanUp? We don't even need to defer VerifyNone! @sywhang