fix test_string_boundaries

A sign, a space or an alternate form parsed fine on a string spec and
then had no effect:

    >>> format('ab', '+')
    'ab'
    >>> f"{'ab':#5}"
    'ab   '

CPython raises ValueError for all three. format_string already refused z
and an explicit '=' alignment with the same family of messages, so the
three checks go next to those, in the order CPython reports them: sign,
then z, then the alternate form, then the alignment.

Assisted-by: Claude Code:claude-opus-5