General cleanup of the testbench. (#1160)
General cleanup of the testbench. (#1160)

The testbench needed some TLC. I cleaned up missing return types, which
are super helpful in my IDE, removed redundant comments, use
lookup_bucket() to find buckets and raise the right error when not
found, and generally removed warnings from the code.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp