pubsub: limit goroutines in benchwrapper to 1
pubsub: limit goroutines in benchwrapper to 1

Benchmarker is currently failing as the fake does not support multiple
subscriber clients yet.

Change-Id: I1cd3c9ca64292ecfbc27a7bf1013dcc6a57e5198
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/53511
Reviewed-by: Tyler Bui-Palsulich <b41e4aa57d2ad7eb1b29835a1b483e209dcc3c0e@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go