fix: make test not flaky (googleapis/google-cloud-cpp-common#143)
fix: make test not flaky (googleapis/google-cloud-cpp-common#143)

No need to wait for X milliseconds on the future. We are going to block
on the same future later on the test, that is really what we want to
verify, that the function is called "eventually".
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp