This commit removes the usage of MockGoogleCloudStoragePlugin in
GoogleCloudStorageBlobStoreRepositoryTests and replaces it by a
HttpServer that emulates the Storage service. This allows the repository
tests to use the real Google's client under the hood in tests and will allow
us to test the behavior of the snapshot/restore feature for GCS repositories
by simulating random server-side internal errors.
The HTTP server used to emulate the Storage service is intentionally simple
and minimal to keep things understandable and maintainable. Testing full
client options on the server side (like authentication, chunked encoding
etc) remains the responsibility of the GoogleCloudStorageFixture.
28974b5723