form.submit should verify it's possible to submit the form

I was surprised to learn today that I had made a form which was impossible to submit. I had used webtest to verify that the form was submittable by calling the "form.submit()" method.

If the button referenced in submit() doesn't exist, or if it doesn't actually submit the form, an error should be raised.

For example, this test could be added to tests/test_forms.py:

    def test_incorrect_button_name(self):
        form = self.callFUT(formid='multiple_buttons_form')
        with self.assertRaises(ValueError):
            form.submit("button_that_does_not_exist")

Then another test is needed where the button exists, but doesn't actually submit the form.

Start with tests/test_forms.py and the existing form.submit behavior. Add coverage for a missing button and for a button that does not submit the form, then verify both cases raise ValueError and the existing form tests still pass.