torture: Remove "Failed to add ttynull console" false positive

Commit 757055ae ("init/console: Use ttynull as a fallback when
there is no console") results in the string "Warning: Failed to add
ttynull console. No stdin, stdout, and stderr for the init process!"
appearing on the console, which the rcutorture scripting interprets as
a warning, which causes every rcutorture run to be flagged.  However,
the rcutorture init process never attempts to do any I/O, and thus does
not care that it has no stdin, stdout, or stderr.

This commit therefore causes the rcutorture scripting to ignore this
message.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>