MXS-2300: Add history pruning test

The test checks that the session command history pruning works as
expected.

maxscale-system-test/mariadb_nodes.h

@@ -316,6 +316,13 @@ public:
     int         get_server_id(int index);
     std::string get_server_id_str(int index);
 
+    /**
+     * Get server IDs of all servers
+     *
+     * @return List of server IDs
+     */
+    std::vector<int> get_all_server_ids();
+
     /**
      * @brief Execute 'mysqladmin flush-hosts' on all nodes
      * @return 0 in case of success