Comment (by Roland Haas):

Note that reading https://docs.github.com/en/pages/configuring-a-custom-domain-for-your-github-pages-site/managing-a-custom-domain-for-your-github-pages-site one actually needs a CNAME entry that points tests.einsteintoolkit.org to einsteintoolkit.github.io (and then GtiHub settings to link things to the repo in question).

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2672/obtain-dns-enty-to-point-https