Treat all warnings as errors during core tests (?)

[frostedoyster]

This treats all warnings as errors during tests, extracted from #315. I don't think it's a good idea to have this for now because it makes it easier for things to break and it will slow down our development

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📚 Documentation preview 📚: https://metatrain--323.org.readthedocs.build/en/323/

[PicoCentauri]

It is only treating warnings as errors in the core part and not in the architectures. Why do you think it will slow down the development?

I think warnings that will be replaced to the users may confuse them even more compared to developers which should fix them.

Also, usual deprecation warnings which should be fixed by us usually get ignored until the feature is gone and errors are raised. I think it only shifts fixing issues to later stages but the work has to be done at some point. See for examples #316. I think we reached a stage were we can treat them as errors.