selftests: forwarding: local_termination: Down ports on cleanup

[ Upstream commit 65a3cce43d5b4c53cf16b0be1a03991f665a0806 ]

This test neglects to put ports down on cleanup. Fix it.

Fixes: 90b9566a

 ("selftests: forwarding: add a test for local_termination.sh")
Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/bf9b79f45de378f88344d44550f0a5052b386199.1724692132.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>