spec/models/project_spec.rb fails when using gitaly master

As of this writing, gitaly master has some changes where it'll return a not found error when unlinking a repository from a pool. Introduced in gitaly!1151 (merged).

The spec for Project#leave_pool_repository in spec/models/project_spec.rb fails when the gitaly server version being used is master (or any branches rebased from master). It's because the object pool isn't being created in the spec so it won't be found.