Fix flaky spec using Capybara::Node#synchronize

This will keep clicking the Filter button until the promotions menu
appears. The problem this fixes is that a few milliseconds after
navigation, the JS controller that switches the filter bar out might not
have loaded.

Alternative fixes here:
solidusio#5782
solidusio#5783

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

@@ -12,10 +12,16 @@
   it "lists products", :js do
     visit "/admin/orders"
 
-    click_button "Filter"
+    # Keep on clicking until the Promotions menu appears
+    page.document.synchronize do
+      click_button "Filter"
+
+      within("div[role=search]") do
+        expect(page).to have_content("Promotions")
+      end
+    end
 
     within("div[role=search]") do
-      expect(page).to have_content("Promotions")
       find(:xpath, "//summary[normalize-space(text())='Promotions']").click
     end
     check "10OFF"