test: hush: Fix variable expansion tests for modern hush

Modifies the expected result for modern hush.
Indeed, there were bugs in actual U-Boot hush which were fixed in upstream
Busybox.
As modern hush is based on upstream Busybox, these bugs no longer exist.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Francis Laniel <francis.laniel@amarulasolutions.com>