tc-testing: fix arg to ip command: -s -> -n

Fixes: 31c2611b ("selftests: Introduce a new test case to tc testsuite")
Fixes: 76b903ee ("selftests: Introduce tc testsuite")
Signed-off-by: Brenda J. Butler <bjb@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>