cloud/storage/gcs: disable test unless magic flag is set
cloud/storage/gcs: disable test unless magic flag is set

These tests only run if one has been authorized with gcloud login and
created a bucket called upspin-test-scratch. Hence, the test is disabled
unless someone really wants to run it. Instructions on how to run are
given on every run.

Fixes #162

Tested both ways.

No new lint or vet errors.

Change-Id: I8c7ab7c86eb40fa6b567dddc279328544f067353
Reviewed-on: https://upspin-review.googlesource.com/7804
Reviewed-by: David Presotto <516b9783fca517eecbd1d064da2d165310b19759@google.com>

upspin/upspin,upspin/upspin