Add a test for `service.variables`

Add a test in spec/lib/gitlab/ci/config/entry/service_spec.rb for #value for the case where the service has variables:

Probably something like:

    context 'when configuration has variables options' do
      let(:config) { { name: 'postgresql:9.5', variables: { POSTGRES_DB: 'test' } } }

      describe '#valid?' do
        it 'is valid' do
          expect(entry).to be_valid
        end
      end

      describe '#value' do
        it "returns value" do
          expect(entry.value).to eq(
            name: 'postgresql:9.5',
            variables: [{
              key: 'POSTGRES_DB',
              value: 'test'
            }]
          )
        end
      end
    end

/cc @kushalpandya @alberts-gitlab