mariabackup.xb_rocksdb_datadir fails in buildbot with Unknown storage engine 'ROCKSDB'

mariabackup.xb_rocksdb_datadir           w2 [ fail ]

        Test ended at 2019-02-18 21:27:08

CURRENT_TEST: mariabackup.xb_rocksdb_datadir

[00] 2019-02-18 21:27:07 /usr/bin/mariabackup based on MariaDB server 10.2.23-MariaDB debian-linux-gnu (ppc64le)

[01] 2019-02-18 21:27:07 Moving ibdata1 to /dev/shm/var/2/mysqld.1/data//ibdata1

[01] 2019-02-18 21:27:07         ...done

...

[01] 2019-02-18 21:27:07         ...done

[00] 2019-02-18 21:27:07 completed OK!

mysqltest: At line 31: query 'SELECT * FROM t' failed: 1286: Unknown storage engine 'ROCKSDB'

The result from queries just before the failure was:

CREATE TABLE t(i INT) ENGINE ROCKSDB;

INSERT INTO t VALUES(1);

# xtrabackup backup

INSERT INTO t VALUES(2);

# xtrabackup prepare

SELECT * FROM t;