tests: Turn off appctl poll_loop logging for long output.

tests: Turn off appctl poll_loop logging for long output.

One of the VMware internal autobuilder builds failed due to extraneous
logging in these tests of the form:

   2014-07-28T21:11:07Z|00001|poll_loop|INFO|wakeup due to [POLLIN] on fd 3
   (...) at lib/stream-fd-unix.c:124 (93% CPU usage)

I think this must be because these tests have tons of output and so on a
loaded machine it can take some CPU to pull it down.  We don't want to fail
for that reason, so disable this logging.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ansis Atteka <aatteka@nicira.com>