selftests: fib_tests: sleep after changing carrier

sleep for a second after setting carrier down to allow linkwatch
to propagate the change to the routing stack via netdev_state_change.
As it stands there is a race setting carrier down on the dummy
device and then checking the linkdown flag in the routes.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>