Ensure westeros_baseline tutorial is consistent with make_westeros

> 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_

Start by locating the baseline tutorial and the `make_westeros` entry point; no file or test path is named in the issue. Read how each produces its `OBJ` value, then determine how a requirement could compare them. Done means a test or documented check exposes inconsistency between the solved `make_westeros` result and the baseline tutorial.

Written by the indexing model from the issue text.