[Matrix]: add unit test on append of mem vector.
[Matrix]: add unit test on append of mem vector.

zheng-da/FlashX,icoming/FlashX,icoming/FlashX,icoming/FlashGraph,icoming/FlashGraph,zheng-da/FlashX,icoming/FlashGraph,flashxio/FlashX,icoming/FlashGraph,zheng-da/FlashX,flashxio/FlashX,icoming/FlashX,flashxio/FlashX,icoming/FlashGraph,icoming/FlashX,flashxio/FlashX,icoming/FlashX,zheng-da/FlashX,zheng-da/FlashX,flashxio/FlashX,flashxio/FlashX