bigquery: use the token source in integration test
bigquery: use the token source in integration test

Change-Id: I233a9e2113695b2ed3f1719b27b25b0a00b34ef1
Reviewed-on: https://code-review.googlesource.com/5452
Reviewed-by: Dave Day <4cba7a21e1f1fc896442779dcb55aeda6df97dfe@golang.org>

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