support/testing: fs/ext: add a negative test for the "extent" feature

We only have a positive test for it, in ext4. Let's have a negative
one as well.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>