Make the 'User comments on a snippet' system test more robust

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

spec/features/projects/snippets/user_comments_on_snippet_spec.rb

@@ -13,6 +13,9 @@ describe 'Projects > Snippets > User comments on a snippet', :js do
     sign_in(user)
 
     visit(project_snippet_path(project, snippet))
+
+    # Snippet's content is loaded async, we wait for it before we try to click anything
+    wait_for_requests
   end
 
   it 'leaves a comment on a snippet' do
@@ -34,7 +37,8 @@ describe 'Projects > Snippets > User comments on a snippet', :js do
   end
 
   it 'has zen mode' do
-    find('.js-zen-enter').click
+    click_button 'Go full screen'
+
     expect(page).to have_selector('.fullscreen')
   end
 end