EC: Block allocation should not be stripped across the EC group

In HDDS-6452, we have discussed the unit of EC block allocation.

In each container replica, the minimum allocation unit should be scmBlockSize
instead of scmBlockSize / dataNum.
For each key, the minimum allocation unit should be scmBlockSize * dataNum,
instead of scmBlockSize.

So the allocation logic in TestWritableECContainerProvider should be updated.