t/perldoc.t: fix TODO logic in todofaster
C reset_once: omit SvLEN,SvCUR > 5.17.6

since 5.17.6 the SvLEN stores RX_WRAPPED(rx), and rx is at SvANY and PVX.
t/testc.sh 75 (#71) only passes with 5.14.4-nt. #188 passes >5.10