test: expand test coverage around forms (#4365)

* Rework all form tests to run both with and without JavaScript. This way we
  can better detect/prevent regressions or inconsistencies between how <form>
  and <Form> work.
* Dedupe and expand form method tests to ensure we cover all supported Form
  methods and submitter formMethods
* Expand coverage around form serialization (tree order, image submit buttons,
  files in URL-encoded payloads)
* Conditionally mark tests as failing (via test.fail):
   * Non-get/post <Form> methods with JavaScript disabled (#4420)
   * Form serialization problems with JavaScript enabled (#4342)