The test seems to be broken for a while[1]. The cause for this is that in gitea 1.14 the create-user command got renamed to user create.
create-user
user create
[1] https://hydra.nixos.org/build/151092299
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"su -l gitea -c 'GITEA_WORK_DIR=/var/lib/gitea gitea admin create-user "
"su -l gitea -c 'GITEA_WORK_DIR=/var/lib/gitea gitea admin user create "