Help restoring a mongodump 3.4 backup into a different instance
Answer is  

mongorestore --port 27017 --authenticationDatabase admin -u my_admin_user -p password --gzip /root/mongodb_backup_full_data_dump/


So, you just tell the directory (basedir) where "all" backups are. mongorestore will "restore" those files (in those sub-directories) to databases (come from directory name). 

SO, if you change that "test_db" directory name (mv test_db other_test_db) to "other_test_db", it will restore those files to "other_test_db"