Ensure westeros_baseline tutorial is consistent with make_westeros

[glatterf42]

          > Should we bake this into a test requirement, that the `OBJ` value of a solved `make_westeros` needs to be consistent with the `OBJ` from the baseline tutorial? Maybe not with this PR, but in general this would expose tacit knowledge.

Yes, this would be very good to do—as you say, in a separate PR.

Originally posted by @khaeru in https://github.com/iiasa/message_ix/pull/815#discussion_r1611250938