tests: Add delay in NetFlow unit tests before killing processes.

tests: Add delay in NetFlow unit tests before killing processes.

At the end of the NetFlow unit tests we warp time to force any
remaining flows to expire and then immediately kill OVS and the
collector.  However, this creates a race where sometimes these
processes are killed before the last records are sent or collected.
It's possible to force OVS to go through the run loop one last time
before exiting but it's harder to enforce that the collector receives
the packet.  This simply avoids the problem by adding a 1 second delay
before killing the processes, which should be more than enough time.

Signed-off-by: Jesse Gross <jesse@nicira.com>