tests: Fix up "ofproto-dpif - ofproto-dpif-monitor 1".

Commit 1335a8d578b03e (tests: Fix race condition waiting for monitor
thread.) fixed a race condition in a test.  Commit 8ba0a5227f6 (ovs-thread:
Make caller provide thread name when creating a thread.) slightly changed
the output that the test checked, breaking the test.  However, I was used
to the test occasionally failing due to the race (not realizing that the
race had been fixed) so I applied the commit anyway.

This commit fixes the broken test.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>