cloud/storage/gcs: add flag to name the test bucket
cloud/storage/gcs: add flag to name the test bucket

Tests pass.

Change-Id: Ib573902edd0aa49ff5246bec32e73f7263d8871c
Reviewed-on: https://upspin-review.googlesource.com/7807
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@google.com>

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