Update view for testability

ee/app/views/shared/milestones/_burndown.html.haml

@@ -12,11 +12,11 @@
       .btn-group.js-burndown-data-selector
         %button.btn.btn-sm.btn-primary{ data: { show: 'count' } }
           Issues
-        %button.btn.btn-sm.btn-primary.btn-inverted{ data: { show: 'weight' } }
+        %button.btn.btn-sm.btn-primary.btn-inverted{ data: { show: 'weight', qa_selector: 'weight_button' } }
           Issue weight
   .burndown-chart{ data: { start_date: burndown.start_date.strftime("%Y-%m-%d"),
     due_date: burndown.due_date.strftime("%Y-%m-%d"),
-    burndown_events_path: expose_url(burndown_endpoint) } }
+    burndown_events_path: expose_url(burndown_endpoint), qa_selector: 'burndown_chart' } }
 
 - elsif show_burndown_placeholder?(milestone, warning)
   .burndown-hint.content-block.container-fluid