Fix test in t/overload_cross_class.t. Fixes (eg): https://www.cpantesters.org/cpan/report/3d47714c-454f-11f0-9a27-d5266e8775ea
Ensure that overloaded <=> operations return one of 1, -1, 0, and undef. Required some amendments to GMPq.xs, and some rewriting of parts of some test scripts. Issue raised by https://rt.cpan.org/Ticket/Display.html?id=168076 .