MariaRocks: run rocksdb testsuite with --default-storage-engine=rocksdb

Most tests use CREATE TABLE ... ENGINE=ROCKSB, but there are some
exceptions: rpl_savepoint, rpl_row_stats.
In order to avoid any "oh we are using the wrong storage engine"
surprises, set the default for the whole testsuite.

storage/rocksdb/mysql-test/rocksdb/suite.opt

---ignore-db-dirs=.rocksdb --plugin-load=$HA_ROCKSDB_SO
+--ignore-db-dirs=.rocksdb --plugin-load=$HA_ROCKSDB_SO --default-storage-engine=rocksdb