Similarly to what has been done in for the repository-s3 plugin,
this commit moves the fixture test into a dedicated
repository-gcs/qa/google-cloud-storage project.
It also exposes some environment variables which allows to
execute the integration tests against the real Google Cloud
Storage service. When the environment variables are not
defined, the integration tests are executed using the fixture
added in #28788. Related to #29349.
Tanguy Leroux