clustermd_tests: add test case to test grow_add against cluster-raid1

clustermd_tests: add test case to test grow_add against cluster-raid1

01r1_Grow_add: It contains 3 kinds of growing array.
1. 2 active disk in md array, grow and add new disk into array.
2. 2 active and 1 spare disk in md array, grow and add new disk
   into array.
3. 2 active and 1 spare disk in md array, grow the device-number
   and make spare disk as active disk in array.

Signed-off-by: Zhilong Liu <zlliu@suse.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>