XS::APItest: Flatten src for utf16_to_utf8_reversed

utf16_to_utf8_reversed (the C function) modifies its input, so the XS
function of the same name should flatten the input to avoid modifying
shared buffers.

Brought to you by PERL_DEBUG_READONLY_COW.
COW documentation

plus read-only documentation, since hysterically the two are
intertwined.