pubsub: fix test breakage due to go 1.6 time.Duration.String
pubsub: fix test breakage due to go 1.6 time.Duration.String

Before 1.7, the zero duration printed as "0" instead of "0s".
Unfortunately, this broke a new test which compares duration strings.

Change-Id: I2361a75b7a4df4aeb9f50089eb068c15902379c7
Reviewed-on: https://code-review.googlesource.com/10030
Reviewed-by: Ross Light <dfccc06f14414bff5a59be7fc90abf4404b47fc8@google.com>

michael-grosshans/google-cloud-go,michael-grosshans/google-cloud-go