Add tests for issue prioritization

spec/features/projects/labels/issues_sorted_by_priority_spec.rb

+require 'spec_helper'
+
+feature 'Issue prioritization', feature: true do
+
+  let(:user)    { create(:user) }
+  let(:project) { create(:project, name: 'test', namespace: user.namespace) }
+
+  # According to https://gitlab.com/gitlab-org/gitlab-ce/issues/14189#note_4360653
+  context 'when assigned with prioritized labels' do
+    scenario 'Are sorted properly' do
+      login_as user
+
+      label_1 = create(:label, title: 'label_1', priority: 1)
+      label_2 = create(:label, title: 'label_2', priority: 2)
+      label_3 = create(:label, title: 'label_3', priority: 3)
+      label_4 = create(:label, title: 'label_4', priority: 4)
+      label_5 = create(:label, title: 'label_5') # no priority
+
+      project.labels << label_1
+      project.labels << label_2
+      project.labels << label_3
+      project.labels << label_4
+      project.labels << label_5
+
+      issue_1 = create(:issue, title: 'issue_1', project: project)
+      issue_2 = create(:issue, title: 'issue_2', project: project)
+      issue_3 = create(:issue, title: 'issue_3', project: project)
+      issue_4 = create(:issue, title: 'issue_4', project: project)
+      issue_5 = create(:issue, title: 'issue_5', project: project)
+      issue_6 = create(:issue, title: 'issue_6', project: project)
+      issue_7 = create(:issue, title: 'issue_7', project: project)
+      issue_8 = create(:issue, title: 'issue_8', project: project)
+
+      # Assign labels to issues disorderly
+      issue_4.labels << label_1
+      issue_3.labels << label_2
+      issue_5.labels << label_3
+      issue_2.labels << label_4
+      issue_1.labels << label_5
+      issue_6.labels << label_5
+      issue_7.labels << label_5
+      issue_8.labels << label_5
+
+      visit namespace_project_issues_path(project.namespace, project, sort: 'priority')
+
+      # Ensure we are indicating that issues are sorted by priority
+      expect(page).to have_selector('.dropdown-toggle', text: 'Priority')
+
+      page.within('.issues-list') do
+        expect(find('> li:nth-of-type(1)')).to have_content('issue_4')
+        expect(find('> li:nth-of-type(2)')).to have_content('issue_3')
+        expect(find('> li:nth-of-type(3)')).to have_content('issue_5')
+        expect(find('> li:nth-of-type(4)')).to have_content('issue_2')
+
+        # the rest should be at the bottom
+        expect(find('> li:nth-of-type(5)')).to have_content('issue_7')
+        expect(find('> li:nth-of-type(6)')).to have_content('issue_8')
+        expect(find('> li:nth-of-type(7)')).to have_content('issue_1')
+        expect(find('> li:nth-of-type(8)')).to have_content('issue_6')
+      end
+    end
+  end
+end