tc-testing: flush gact actions on test teardown

Test 6fb4 creates one mirred and one pipe action, but only flushes mirred
on teardown. Leaking pipe action causes failures in other tests.

Add additional teardown command to also flush gact actions.

Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>