tc-testing: Add test cases for police and skbmod

Add basic unit tests for police and skbmod actions in tc.

Signed-off-by: Lucas Bates <lucasb@mojatatu.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>