I've tested this a bit. The reason why you cannot access as root the default database is that in Debian we now how unix socket authentication enabled by default, and the default account allows 'root' to access mysqld via socket. So running your mysqladmin as sudo works, but that of course defeats the purpose of your test deamon run with only normal user priviledges. See https://mariadb.com/kb/en/mariadb/unix_socket-authentication-plugin/
To be able to access your test database anyway, use --skip-grant-tables to circumvent authentication (which is not needed on a test database, right?). /usr/sbin/mysqld --no-defaults --socket=/tmp/tmp.WiS7YFCukw/mysql.sock --datadir=/tmp/tmp.WiS7YFCukw --skip-networking --skip-grant-tables This is not a bug in packaging, but instead of marking this invalid, I'll leave the issue open for the sake of documentation.
