selftests: fib_nexthops: Make the test more robust

Rarely some of the test cases fail. Make the test more robust by increasing
the timeout of ping commands to 5 seconds.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>