deparse.t: mask STDERR noise on lex sub TODO test

A Deparse TODO test which tries to deparse lexical subs, produce a bunch of
"unexpected OP_CLONE" etc on STDERR (since it doesn't handle those new ops
yet). Capture and skip those warnings for now.

Also, add 'no warnings "experimental::lexical_subs"'
to stop a warning about using an experimental feature.