tests/ofproto-dpif.at: Add delay before stopping time.

Add 2 second sleep before time stop to allow internal ports to
reconnect in case the initial connection attempt failed.

Bond tests sometimes persistently fail without these delays.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>