Add helper for mocking/stubbing form.submit

JSDOM doesn't implement the submit method on form
elements. It does have a submit method but this
just fires a 'not implemented' error.

We need to spy on form submissions fired by
clicking on the submit button but can't because
this event calls the submit method internally so
spying on `form.submit` doesn't work.

This adds a helper which spies on the internal
method that is actually called. When JSDOM
implements the submit method properly this should
be removed.

tests/javascripts/support/helpers/forms.js

@@ -0,0 +1,33 @@
+
+// helper for spying on the submit method on a form element
+// JSDOM's implementation of submit just wraps a 'not implemented' error so we need to mock that to track calls to it
+//
+// * Remove when JSDOM implements submit on its form elements *
+//
+// elements in JSDOM have a public API and a private implementation API, only used by internal code
+// For form elements, these are instances of the following classes:
+// - HTMLFormElement
+// - HTMLFormElementImpl
+//
+// form elements link to their implementation instance via a symbol property
+// this spies on the submit method of the implementation instance for a form element and mocks it to prevent 'not implemented' errors
+function spyOnFormSubmit (jest, form) {
+
+  const formImplementationSymbols = Object.getOwnPropertySymbols(form).filter(
+    symbol => form[symbol].constructor.name === 'HTMLFormElementImpl'
+  );
+
+  if (!formImplementationSymbols.length) {
+    throw Error("Error mocking form.submit: symbol reference to HTMLFormElementImpl instance not found on form element");
+  }
+
+  const HTMLFormElementImpl = form[formImplementationSymbols[0]];
+
+  const submitSpy = jest.spyOn(HTMLFormElementImpl, 'submit')
+
+  submitSpy.mockImplementation();
+  return submitSpy;
+
+};
+
+exports.spyOnFormSubmit = spyOnFormSubmit;