the idea of main.failed_auth_unixsocket was to have existing
user account (root) authenticate with unix_socket, then login with
non-existent user name, Non-existent user name forces the server
to perform the authentication in the name of some random existing
user. But it must still fail at the end, as the user name is wrong.

In 10.4 a second predefined user was added, mariadb.sys, so root
is not the only user in mysql.global_priv and unix_socket auth
must be forced for all existing user accounts, because we cannot
know what user account the server will randomly pick for non-existing
user auth.