MDEV-12285: MariaRocks: "[ERROR] mysqld: Deadlock ..." messages in server stderr

The mssages are caused by log_warnings=2.
Set log_warnings=1 for the rocksdb test suite.

storage/rocksdb/mysql-test/rocksdb/my.cnf

@@ -11,3 +11,7 @@ loose-rocksdb_lock_wait_timeout=1
 loose-rocksdb_strict_collation_check=0
 
 loose-rocksdb-flush-log-at-trx-commit=0
+
+# The following is to get rid of the harmless 
+# "Deadlock found when trying to get lock" errors, see MDEV-12285.
+log-warnings=1