Add tests for DeploymentSerializer class

We're calling DeploymentSerializer#represent_concise on DeploymentsController but we got no tests for this method.

Lack of tests caused some overhead to understand if this method needs an User instance or not (we were previously passing user unnecessarily for creating an instance of DeploymentSerializer on DeploymentsController - see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11254)