Work around inconsistent button click in promotion order spec

Apparently, `find_button(...).click` will not consistently wait for the
Javascript imported via importmaps to be loaded before execution. This
adds a helper that executes a bit of Javascript that adds some data to
the body element of the page after waiting for a minute amount of time,
and then waits for this data to actually appear in the DOM.

See https://gist.github.com/adrienpoly/862846f5882796fdeb4fc85b260b3c5a
for a similar solution. This unfortunately adds code to the production
build that's only needed for testing.

admin/lib/solidus_admin/testing_support/feature_helpers.rb

@@ -29,6 +29,15 @@ def find_row_checkbox(text)
       def select_row(text)
         find_row_checkbox(text).check
       end
+
+      def ensure_js_is_ready
+        page.execute_script <<~JS
+          requestAnimationFrame(() => {
+            document.querySelector("body").dataset.jsLoaded = true
+          }
+        JS
+        expect(page).to have_css('[data-js-loaded="true"]')
+      end
     end
   end
 end

legacy_promotions/spec/features/solidus_admin/orders/index_spec.rb

@@ -12,6 +12,8 @@
   it "lists products", :js do
     visit "/admin/orders"
 
+    ensure_js_is_ready
+
     click_button "Filter"
 
     within("div[role=search]") do