selftests: net: check veth carrier after changing channel counts

Extend veth.sh to check that changing channel counts preserves carrier
state. When only one end is up, the running end must not report LOWER_UP.

Cover up/up and both mixed up/down combinations. Resize RX and TX queues
in both directions on each endpoint, and check carrier after a rejected
out-of-range request as well. Create the pair with two queues explicitly
so the carrier cases also run on single-CPU systems.

Reuse the existing namespace setup and cleanup. Run each carrier case
in a subshell to keep its variables separate from the other tests, and
accumulate failures in the caller. Only check LOWER_UP on running
interfaces, where the flag exposes carrier state.

Tested-by: Hangbin Liu <liuhangbin@kylinos.cn>
Reviewed-by: Hangbin Liu <liuhangbin@kylinos.cn>
Signed-off-by: Xia Yan <yana06070@163.com>
Link: https://patch.msgid.link/20261029024259.3700-2-yana06070@163.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>