selftests: net: veth: add tests for set_channel

Simple functional test for the newly exposed features.

Also add an optional stress test for the channel number
update under flood.

RFC v1 -> RFC v2:
 - add the stress test

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>