How to test Gitaly while it is optional in gitlab

There are tests that need to work with gitaly on and off:

For example, from spec/models/repository_spec.rb:

  describe '#branch_names_contains' do
    subject { repository.branch_names_contains(sample_commit.id) }

    it { is_expected.to include('master') }
    it { is_expected.not_to include('feature') }
    it { is_expected.not_to include('fix') }
  end

When https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9291 gets merged, depending on the value of Gitlab::GitalyClient.feature_enabled? for the test environment (which depends in turn on ENV variables), that test will rely on a) Gitlab::Git::Repository, or b) Gitlab::GitalyClient::Ref`.

Other tests like

  describe '#root_ref' do
      it 'caches the output' do
        expect(repository.raw_repository).to receive(:root_ref).
          once.
          and_return('master')

        repository.root_ref
        repository.root_ref
      end
  end

directly reference the underlying component, so they would have to be changed to two cases, one which references Gitlab::Git::Repository, and another which references Gitlab::GitalyClient::Ref.