tc-testing: add simple action change test

Use act_simple to verify that action created with 'tc actions change'
command exists after command returns. The goal is to verify internal action
API reference counting to ensure that the case when netlink message has
NLM_F_REPLACE flag set but action with specified index doesn't exist is
handled correctly.

Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>