load -q option doesn't work

[Jelmer Vernooij]

  affects testrepository

echo foo | testr load -q

results in foo being output by "testr load";

while the --help says -q is supposed to :

Turn off output other than the primary output for a command and any
errors.

[Robert Collins (lifeless)]

Ah, so non subunit stuff is escaping - we need to set
passthough_stream to a DiscardStream() in this case.

Hmm, we may find a flip-flop where folk want -q *and* editing
so lets do this on -qq