tests: Avoid race condition in flow monitoring test.

tests: Avoid race condition in flow monitoring test.

The execution of "ovs-appctl -t ovs-ofctl ofctl/send" could happen before
ovs-appctl received and printed the
    NXST_FLOW_MONITOR reply (xid=0x0):
     event=ADDED table=0 cookie=0 in_port=2 actions=output:1
msesage, which could cause the
    send: OFPT_FLOW_MOD: DEL priority=0 actions=drop
message to be printed before rather than after the NXST_FLOW_MONITOR reply.

This commit fixes the problem by inserting a serializing barrier.

Reported-by: Jarno Rajahalme <jarno.rajahalme@nsn.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>