net: mctp: test: Add tests for gateway routes

Add a few kunit tests for the gateway routing. Because we have multiple
route types now (direct and gateway), rename mctp_test_create_route to
mctp_test_create_route_direct, and add a _gateway variant too.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Link: https://patch.msgid.link/20250702-dev-forwarding-v5-14-1468191da8a4@codeconstruct.com.au


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