tc-testing: initial version of tunnel_key unit tests

Create unittests for the tc tunnel_key action.

v2:
For the tests expecting failures, added non-zero exit codes in the
teardowns. This prevents those tests from failing if the act_tunnel_key
module is unloaded.
Signed-off-by: Keara Leibovitz <kleib@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>