selftests: net: local_termination: Wait for interfaces to come up

It seems that most of the tests prepare the interfaces once before the test
run (setup_prepare()), rely on setup_wait() to wait for link and only then
run the test(s).

local_termination brings the physical interfaces down and up during test
run but never wait for them to come up. If the auto-negotiation takes
some seconds, first test packets are being lost, which leads to
false-negative test results.

Use setup_wait() in run_test() to make sure auto-negotiation has been
completed after all simple_if_init() calls on physical interfaces and test
packets will not be lost because of the race against link establishment.

Fixes: 90b9566aa5cd3f ("selftests: forwarding: add a test for local_termination.sh")
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Link: https://patch.msgid.link/20251106161213.459501-1-alexander.sverdlin@siemens.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>