Consolidate _Client and _Connection test shims

[dhermes]

See https://github.com/GoogleCloudPlatform/gcloud-python/pull/1145/files#r40022552

Running git grep -l _Client and git grep -l _Connection turns up a lot of results.

In keeping with @tseaver philosophy, we should avoid added more than the basic functionality common to all tests using this pattern.

[lukesneeringer]

This seems obselete. Reopen if not.