net: mctp: test: Add extaddr routing output test

Test that the routing code preserves the haddr data in a skb through an
input route operation.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Link: https://patch.msgid.link/20250702-dev-forwarding-v5-6-1468191da8a4@codeconstruct.com.au


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