tests/futility/test_dump_fmap: Allow easier expected output updates

Add a global variable UPDATE_MODE to allow updating the expected output
files when set to 1.

For consistency with other futility test scripts, replace `cmp` with
`diff`.

BUG=none
TEST=make runfutiltests
BRANCH=none

Change-Id: Id42be74fcea25cbac5c4b7a3fc3dfe4cb874b76e
Signed-off-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/6342557
Reviewed-by: Julius Werner <jwerner@chromium.org>