With mysqldump it is not possible by the parameters to exclude individual databases. However, the database can be easily queried from the information_schema and this makes an exclude.
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mysqldump --databases `mysql --skip-column-names -e "SELECT GROUP_CONCAT(schema_name SEPARATOR ' ') FROM information_schema.schemata WHERE schema_name NOT IN ('mysql','performance_schema','information_schema','db_test');"` >/dump.sql