Write unit tests for the CloudStorageRetryHandler class

[droazen]

We recently extracted a simple CloudStorageRetryHandler class to keep track of retries/reopens in google-cloud-nio. This class needs unit tests, as per the discussion in #2281

[droazen]

@jean-philippe-martin or myself can take this on when we have time.

[anthmgoogle]

There is a merged PR. Does that mean this can be closed now?