mariabackup.unsupported_redo fails in buildbot with wrong result code

mariabackup.unsupported_redo 'innodb'    w2 [ fail ]

        Test ended at 2018-03-26 12:49:32

CURRENT_TEST: mariabackup.unsupported_redo

mysqltest: At line 26: command "$XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf  --backup --target-dir=$basedir" succeeded - should have failed with errno 1...

The result from queries just before the failure was:

call mtr.add_suppression("InnoDB: New log files created");

call mtr.add_suppression("InnoDB: Operating system error number .* in a file operation");

call mtr.add_suppression("InnoDB: The error means the system cannot find the path specified");

call mtr.add_suppression("InnoDB: If you are installing InnoDB, remember that you must create directories yourself, InnoDB does not create them");

call mtr.add_suppression("InnoDB: Ignoring tablespace for `test`\\.`t21` because it could not be opened");

call mtr.add_suppression("InnoDB: Cannot open datafile for read-only: ");

call mtr.add_suppression("Table .* in the InnoDB data dictionary has tablespace id .*, but tablespace with that id or name does not exist");

CREATE TABLE t1(i INT PRIMARY KEY auto_increment, a int) ENGINE INNODB;

SET GLOBAL INNODB_FAST_SHUTDOWN = 0;

ALTER TABLE t1 FORCE, ALGORITHM=INPLACE;

# Fails during full backup

 - saving 'D:/win32-debug/build/mysql-test/var/2/log/mariabackup.unsupported_redo-innodb/' to 'D:/win32-debug/build/mysql-test/var/log/mariabackup.unsupported_redo-innodb/'