DEV: ensure bulk action is done before checking for result (#602)

We were sometimes visiting latest before the request happens, the solution is to wait for the modal to have automatically closed as it indicates the request has finished and the state should be the one we expect.

spec/system/bulk_assign_spec.rb

@@ -46,6 +46,7 @@
 
       # Click Confirm
       topic_list_header.click_bulk_topics_confirm
+      expect(assign_modal).to be_closed
 
       # Reload and check that topic is now assigned
       visit "/latest"
@@ -65,6 +66,7 @@
 
       # Click Confirm
       topic_list_header.click_bulk_topics_confirm
+      expect(assign_modal).to be_closed
 
       # Reload and check that topic is now assigned
       visit "/latest"