Add bucket controller tests for reconcileGCPSource

- Introduce mock GCP Server to test the gcp bucket client against mocked
  gcp server results.
- Add tests for reconcileGCPSource().
- Patch GCPClient.BucketExists() to return no error when the bucket
  doesn't exists. This keeps the GCP client compatible with the minio
  client.

Signed-off-by: Sunny <[email protected]>

Author: darkowlzz