testing: selftests: forwarding.config.sample: Add tc flag

Add TC_FLAG value to tests topology.
This flag supposed to be skip_sw/skip_hw which means do not filter by
software/hardware.

This can be useful for adding tests to forwarding directory, and be able
to verify that packets go through the hardware.

When the flag is not set or set to 'skip_hw', tests can still be executed
with veth pairs.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>