Add system test to test form submission

previews/primer/open_project/danger_confirmation_dialog_preview.rb

@@ -41,12 +41,15 @@ def playground(
       end
 
       # @label With form
-      def with_form
-        render(Primer::OpenProject::DangerConfirmationDialog.new(form_arguments: {
-          method: :post,
-          action: "/my-controller/my-action",
-          novalidate: true
-        })) do |dialog|
+      def with_form(route_format: :html)
+        render(Primer::OpenProject::DangerConfirmationDialog.new(
+          title: "Delete dialog",
+          form_arguments: {
+            method: :post,
+            action: generic_form_submission_path(format: route_format),
+            novalidate: true
+          }
+        )) do |dialog|
           dialog.with_show_button { "Click me" }
           dialog.with_confirmation_message do |message|
             message.with_heading(tag: :h2).with_content("Permanently delete?")

test/system/open_project/danger_confirmation_dialog_test.rb

@@ -32,4 +32,19 @@ def test_submit_button_enabled_when_confirmation_check_box_checked
       assert_selector("button[data-submit-dialog-id]:enabled")
     end
   end
+
+  def test_submit_button_submits_form
+    visit_preview(:with_form, route_format: :json)
+
+    click_button("Click me")
+
+    assert_selector(".DangerConfirmationDialog") do
+      check("I understand that this deletion cannot be reversed")
+      find("button[type='submit']").click
+
+      # for some reason the JSON response is wrapped in HTML, I have no idea why
+      response = JSON.parse(find("pre").text)
+      assert_equal "fast_forward", response["value"]
+    end
+  end
 end