Wait for all CA stages to be rendered

When rendering cycle analytics stages, it seems that the frontend first
renders the add stage button and later on populates the stages. When the
stages are populated the button is probably re-rendered which makes the
class change on the button disappear.

ee/spec/features/analytics/cycle_analytics/cycle_analytics_spec.rb

@@ -233,25 +233,32 @@ describe 'Group Cycle Analytics', :js do
   end
 
   describe 'Customizable cycle analytics', :js do
+    let(:button_class) { '.js-add-stage-button' }
+
     context 'enabled' do
       before do
         dropdown = page.find('.dropdown-groups')
         dropdown.click
         dropdown.find('a').click
+
+        # Make capybara wait until all the .stage-nav-item elements are rendered
+        # We should have NUMBER_OF_STAGES + 1 (button)
+        expect(page).to have_selector(
+          '.stage-nav-item',
+          count: Gitlab::Analytics::CycleAnalytics::DefaultStages.all.size + 1
+        )
       end
 
       context 'Add a stage button' do
         it 'is visible' do
-          expect(page).to have_selector('.js-add-stage-button', visible: true)
+          expect(page).to have_selector(button_class, visible: true)
           expect(page).to have_text('Add a stage')
         end
 
         it 'becomes active when clicked' do
-          button_class = '.js-add-stage-button'
-
           expect(page).not_to have_selector("#{button_class}.active")
 
-          page.find(button_class).click
+          find(button_class).click
 
           expect(page).to have_selector("#{button_class}.active")
         end
@@ -259,7 +266,7 @@ describe 'Group Cycle Analytics', :js do
         it 'displays the custom stage form when clicked' do
           expect(page).not_to have_text('New stage')
 
-          page.find('.js-add-stage-button').click
+          page.find(button_class).click
 
           expect(page).to have_text('New stage')
         end