tc-testing: Add test cases for flushing actions

Tests for flushing gact and mirred were missing. This patch
adds test cases to explicitly test the flush of any installed
gact/mirred actions.

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>