tests: Fix race condition waiting for monitor thread.

Occasionally, test #770 "ofproto-dpif - ofproto-dpif-monitor 1" would
fail, because the testsuite looked in the logs for evidence of a thread
being created, but it checked before vswitchd was able to spawn the
thread.

This patch fixes the race by modifying the commands that check for
creation/termination of threads to wait until they see the messages
instead.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Reviewed-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>