Disable rocksdb.shutdown test

It was introduced by this patch in fb/mysql-5.6:
Author: Yoshinori Matsunobu <yoshinori@fb.com>
Date:   Mon Jun 10 14:09:28 2019 -0700

    Extending SHUTDOWN query to support read_only/aborting

    Summary:
    This diff extends SHUTDOWN query to support the following
    features.
    - Aborting with any specified exit code (range is 0..255).
    If nothing is specified or 0 is given, it does default clean
    shutdown. If 1+ is given, exits with the given error code
    immediately. This is helpful to shutting down instance
    even if it is stuck somewhere.

MariaDB doesn't support SHUTDOWN statement or have any other way
to exit the server process.

storage/rocksdb/mysql-test/rocksdb/t/disabled.def

@@ -38,6 +38,8 @@ rocksdb_read_free_rpl_stress :  Read-Free replication is not supported
 blind_delete_rr : Read-Free replication is not supported
 blind_delete_rc : Read-Free replication is not supported
 
+shutdown: requires support for SHUTDOWN statement which calls exit().
+
 ##
 ##  Tests that do not fit MariaDB's test environment. Upstream seems to test
 ##  on boxes with much more RAM and CPU, some tests are more of a stress tests