fix(stl_traffic): track both ports on unidir

Even if the suite asks for unidirectional stateless traffic,
in some circumstances (e.g. reconf tests)
stats from both ports are needed to determine overall loss.
Enabling both ports unconditionally fixes that
without any negative consequences.

Latency info is still gathered only for one direction in unidir tests.

Change-Id: Ifec6603d6103c9981fc0af02a81b0db215ccb77c
Signed-off-by: Vratko Polak <vrpolak@cisco.com>