tc-testing: fix the -l argument bug in tdc.py

This patch fixes a bug in the tdc script, where executing tdc
with the -l argument would cause the tests to start running
as opposed to listing all the known test cases.

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>