Fix build issue with Browser.submitForm
Fix build issue with Browser.submitForm

`jsdom` does not implement `form.submit()`, so call to
`Browser.submitForm` that were not mocked or using a mock form object
were triggering an error along the lines of:

`Error: Not implemented: HTMLFormElement.prototype.submit`

Mock `Browser.submitForm` for all the `postToNewWindow` tests and
manually reset the mock after each test.

Sage/carbon,Sage/carbon,Sage/carbon