t/op/caller.t: Remove hardcoded $warnings::BYTES in favor of using $warnings::BYTES

This was originally hardcoded here in perl-5.8.0-909-g75b6c4c but since
then in numerous subsequent commits starting with v5.13.9-546-g3a32947
we've been bumping this number as we increase the number of warning
bits.

Just use the variable from warnings.pm instead, we're already using
other variables from there.