XS-APItest/t/handy.t: Refactor

XS-APItest/t/handy.t: Refactor

This takes advantage of the Unicode::UCD::prop_invlist() function,
introduced in 5.16, to replace the hard-coded tables of Unicode values
previously used  (and also relying on recent fixes to an undocumented
function from that module ).  It als changes to give better messages if
the evals fail.  Using prop_invlist() allows us to easily extend to all
tests testing of some non-Latin1 code points.