Disable rocksdb.force_shutdown, rocksdb.shutdown is fine

Fix this patch (two csets before):
  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

@@ -38,7 +38,7 @@ 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().
+force_shutdown: requires support for SHUTDOWN statement which calls exit().
 
 ##
 ##  Tests that do not fit MariaDB's test environment. Upstream seems to test