log: Move PayloadBytes tests into grpcstats package

Our log package hosts a test that verifies whether we can correctly log the bytes of the request and response payloads. This logic is not part of the log package though, but is instead implemented by the grpcstats package.

Let's move the test to be hosted in the correct package such that the system under test and the test itself are close together. This also addresses an upcoming import cycle in our tests between the grpcstats and the log package.