storage: readObject method for tests
storage: readObject method for tests

Add a method to simplify some tests.

Change-Id: I6e38ec01d9cfd8af72a45731ad9e1d9aa30ce39c
Reviewed-on: https://code-review.googlesource.com/8991
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

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