Add a feature spec for the performance bar

Signed-off-by: Rémy Coutable <remy@rymai.me>

spec/features/user_can_display_performance_bar_spec.rb

+require 'rails_helper'
+
+describe 'User can display performacne bar', :js do
+  shared_examples 'performance bar is disabled' do
+    it 'does not show the performance bar by default' do
+      expect(page).not_to have_css('#peek')
+    end
+
+    context 'when user press `pb`' do
+      before do
+        find('body').native.send_keys('pb')
+      end
+
+      it 'does not show the performance bar by default' do
+        expect(page).not_to have_css('#peek')
+      end
+    end
+  end
+
+  shared_examples 'performance bar is enabled' do
+    it 'does not show the performance bar by default' do
+      expect(page).not_to have_css('#peek')
+    end
+
+    context 'when user press `pb`' do
+      before do
+        find('body').native.send_keys('pb')
+      end
+
+      it 'does not show the performance bar by default' do
+        expect(page).not_to have_css('#peek')
+      end
+    end
+  end
+
+  context 'when user is logged-out' do
+    before do
+      visit root_path
+    end
+
+    context 'when the gitlab_performance_bar feature is disabled' do
+      before do
+        Feature.disable('gitlab_performance_bar')
+      end
+
+      it_behaves_like 'performance bar is disabled'
+    end
+
+    context 'when the gitlab_performance_bar feature is enabled' do
+      before do
+        Feature.enable('gitlab_performance_bar')
+      end
+
+      it_behaves_like 'performance bar is disabled'
+    end
+  end
+
+  context 'when user is logged-in' do
+    before do
+      login_as :user
+
+      visit root_path
+    end
+
+    context 'when the gitlab_performance_bar feature is disabled' do
+      before do
+        Feature.disable('gitlab_performance_bar')
+      end
+
+      it_behaves_like 'performance bar is disabled'
+    end
+
+    context 'when the gitlab_performance_bar feature is enabled' do
+      before do
+        Feature.enable('gitlab_performance_bar')
+      end
+
+      it_behaves_like 'performance bar is enabled'
+    end
+  end
+end