tc-testing: better check if thing is list

Check if tcase[k] is an instance of a list (is or is derived from list)
instead of checking if it is a list.

This will be useful if the data structures change to be something
that implements list, instead of being an actual list.  In that
case, this code will not have to change.

Signed-off-by: Brenda J. Butler <bjb@mojatatu.com>
Acked-by: Lucas Bates <lucasb@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>