erasure-code: isa encode tests adapted to per chunk alignment

erasure-code: isa encode tests adapted to per chunk alignment

The encode tests use the alignment constraints. It has been changed to
be aligned on a per chunk basis instead of computing a more expensive
object alignement constraint. The test function is modified to take the
change into account but the logic is otherwise unmodified.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>