mariabackup mtr tests don't handle properly option --mysqld='--innodb_data_file_path...'

CURRENT_TEST: mariabackup.full_backup

mysqltest: At line 7: exec of '/home/a/test1/mariadb-environs/_depot/m-tar/10.1.23/bin/mariabackup --defaults-file=/home/a/test1/mariadb-environs/_depot/m-tar/10.1.23/mysql-test/var/my.cnf --backup --target-dir=/home/a/test1/mariadb-environs/_depot/m-tar/10.1.23/mysql-test/var/tmp/backup 2>&1' failed, error: 256, status: 1, errno: 0

Output from before failure:

170506 08:32:26 Connecting to MySQL server host: localhost, user: root, password: set, port: 16000, socket: /home/a/test1/mariadb-environs/_depot/m-tar/10.1.23/mysql-test/var/tmp/mysqld.1.sock

Using server version 10.1.23-MariaDB

/home/a/test1/mariadb-environs/_depot/m-tar/10.1.23/bin/mariabackup based on MariaDB server 10.1.23-MariaDB Linux (x86_64) 

xtrabackup: uses posix_fadvise().

xtrabackup: cd to /home/a/test1/mariadb-environs/_depot/m-tar/10.1.23/mysql-test/var/mysqld.1/data/

xtrabackup: open files limit requested 1024, set to 1024

xtrabackup: using the following InnoDB configuration:

xtrabackup:   innodb_data_home_dir = ./

xtrabackup:   innodb_data_file_path = ibdata1:12M:autoextend

xtrabackup:   innodb_log_group_home_dir = ./

xtrabackup:   innodb_log_files_in_group = 2

xtrabackup:   innodb_log_file_size = 5242880

170506 08:32:26 >> log scanned up to (1632961)

InnoDB: auto-extending data file ./ibdata1 is of a different size 640 pages (rounded down to MB) than specified in the .cnf file: initial 768 pages, max 0 (relevant if non-zero) pages!

xtrabackup: Could not open or create data files.

xtrabackup: If you tried to add new data files, and it failed here,

xtrabackup: you should now edit innodb_data_file_path in my.cnf back

xtrabackup: to what it was, and remove the new ibdata files InnoDB created

xtrabackup: in this failed attempt. InnoDB only wrote those files full of

xtrabackup: zeros, but did not yet use them in any way. But be careful: do not

xtrabackup: remove old data files which contain your precious data!

xtrabackup: error: xb_load_tablespaces() failed witherror code 11