op.c: Fix PERL_DEBUG_READONLY_OPS

I accidentally broke this in 2eaf799e7.
Make Peek.t less sensitive to NO_COW config args

Maybe what I am doing is too clever, but I just take the last Configure
command line and add an S to -DPERL_NO_COW to disable it.  It doesn’t
hurt to make Peek.t more robust, though.