spanner: make timebound test location-independent
spanner: make timebound test location-independent

Use UTC so it doesn't matter where you are when you run the test.

Change-Id: Ie59046e963723bc8ff1407eafa5851048c155f06
Reviewed-on: https://code-review.googlesource.com/10991
Reviewed-by: Chris Broadfoot <0c2851a4f8ab4aa5606e69c0757771dd13dbe686@google.com>

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