Add isolated view spec for pipeline stage partial

spec/views/projects/pipelines/_stage.html.haml_spec.rb

+require 'spec_helper'
+
+describe 'projects/pipelines/_stage', :view do
+  let(:project) { create(:project) }
+  let(:pipeline) { create(:ci_pipeline, project: project) }
+  let(:stage) { build(:ci_stage, pipeline: pipeline) }
+
+  before do
+    assign :stage, stage
+
+    create(:ci_build, name: 'test:build',
+                      stage: stage.name,
+                      pipeline: pipeline)
+  end
+
+  it 'shows the builds in the stage' do
+    render
+
+    expect(rendered).to have_text 'test:build'
+  end
+end