Correct taint.t to skip the truncate test if $Config{d_truncate} is false.

Correct taint.t to skip the truncate test if $Config{d_truncate} is false.

Previously it had a comment, present since 5.003_92, that "There is no feature
test in $Config{} for truncate, so we allow for the possibility that it's
d missing."

This was never correct, as 5.000 had d_truncate, and taint.t had analogous
tests for $Config{d_chown} etc.