Test fixes in rocksdb_sys_vars test suite

- Get the suite to work with dynamically-linked plugin (ha_rocksdb.so)
- Due to the push to keep everything MyRocks-related in storage/rocksdb,
  there is no mysql-test/include/have_rocksdb.* anymore.
  Make a copy of storage/rocksdb/mysql-test/rocksdb/include/have_rocksdb*,
  hopefully these files wont be changed [often].
- Maria-fication of rocksdb_persistent_cache_path test.

storage/rocksdb/mysql-test/rocksdb_sys_vars/include/have_rocksdb.inc

+if (`SELECT COUNT(*) = 0 FROM INFORMATION_SCHEMA.ENGINES WHERE engine = 'rocksdb' AND support IN ('YES', 'DEFAULT', 'ENABLED')`)
+{
+  --skip Test requires engine RocksDB.
+}
+
+--disable_query_log
+# Table statistics can vary depending on when the memtables are flushed, so
+# flush them at the beginning of the test to ensure the test runs consistently.
+set global rocksdb_force_flush_memtable_now = true;
+--enable_query_log

storage/rocksdb/mysql-test/rocksdb_sys_vars/include/have_rocksdb.opt

+--loose-enable-rocksdb
+--loose-enable-rocksdb_global_info
+--loose-enable-rocksdb_ddl
+--loose-enable-rocksdb_cf_options
+--loose-enable_rocksdb_perf_context
+--loose-enable_rocksdb_perf_context_global
+--loose-enable-rocksdb_index_file_map
+--loose-enable-rocksdb_dbstats
+--loose-enable-rocksdb_cfstats
+--loose-enable-rocksdb_lock_info
+--loose-enable-rocksdb_trx
+--loose-enable-rocksdb_locks

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

 !include include/default_my.cnf
 
 [server]
-rocksdb
 skip-innodb
 default-storage-engine=rocksdb
 
 
 sql-mode=NO_ENGINE_SUBSTITUTION
 explicit-defaults-for-timestamp=1
-rocksdb_lock_wait_timeout=1
+loose-rocksdb_lock_wait_timeout=1

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

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

storage/rocksdb/mysql-test/rocksdb_sys_vars/t/rocksdb_persistent_cache_path_basic.test

@@ -10,7 +10,7 @@ CREATE TABLE invalid_values (value varchar(255)) ENGINE=myisam;
 --let $read_only=1
 --let $session=0
 --let $sticky=1
---source suite/sys_vars/inc/rocksdb_sys_var.inc
+--source include/rocksdb_sys_var.inc
 
 DROP TABLE valid_values;
 DROP TABLE invalid_values;