[Form][BUG]Button missing getErrorsAsString() fixes #8084
Debug: Not calling undefined method anymore.
If the form contained a submit button the call would fail and the debug of the form wasn't possible.
Now it will work in all cases.
This fixes #8084

Adding a test for the fix of getErrorAsString on Form.
Was throwing a fatal because of a method that did not exist on
the new element type button.