test: dm: blk: Correct blk_base test case

The blk_base test case creates a USB mass storage block device with
the Sandbox host block device as its parent. This does not make any
sense and causes potential issue, for example if the test case tries
to read/write anything on the USB mass storage block device it will
definitely fail as its parent is not on USB bus at all.

Correct the test case by creating another Sandbox host block device
instead of the USB mass storage one and adjust the case accordingly.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>