Wait for modal before testing its content

Building off of #5993, a few
more specs required a short wait time to give the modal a chance to
load. They were flaky (failing somewhat regularly in CI, but always
passing locally) and this should help remedy that.

admin/spec/features/refund_reasons_spec.rb

@@ -26,7 +26,7 @@
     before do
       visit "/admin/refund_reasons/#{query}"
       click_on "Add new"
-      expect(page).to have_selector("dialog")
+      expect(page).to have_css("dialog", wait: 5)
       expect(page).to have_content("New Refund Reason")
       expect(page).to be_axe_clean
     end
@@ -66,7 +66,7 @@
       Spree::RefundReason.create(name: "Return process")
       visit "/admin/refund_reasons#{query}"
       find_row("Return process").click
-      expect(page).to have_selector("dialog")
+      expect(page).to have_css("dialog", wait: 5)
       expect(page).to have_content("Edit Refund Reason")
       expect(page).to be_axe_clean
     end

admin/spec/features/shipping_categories_spec.rb

@@ -26,7 +26,7 @@
     before do
       visit "/admin/shipping_categories#{query}"
       click_on "Add new"
-      expect(page).to have_selector("dialog")
+      expect(page).to have_css("dialog", wait: 5)
       expect(page).to have_content("New Shipping Category")
       expect(page).to be_axe_clean
     end
@@ -66,7 +66,7 @@
       Spree::ShippingCategory.create(name: "Letter Mail")
       visit "/admin/shipping_categories#{query}"
       find_row("Letter Mail").click
-      expect(page).to have_selector("dialog")
+      expect(page).to have_css("dialog", wait: 5)
       expect(page).to have_content("Edit Shipping Category")
       expect(page).to be_axe_clean
     end