net: mctp: test: Add an addressed device constructor

Upcoming tests will check semantics of hardware addressing, which
require a dev with ->addr_len != 0. Add a constructor to create a
MCTP interface using a physically-addressed bus type.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Link: https://patch.msgid.link/20250702-dev-forwarding-v5-5-1468191da8a4@codeconstruct.com.au


Signed-off-by: Paolo Abeni <pabeni@redhat.com>