erasure-code: test isa encode/decode with various object sizes

erasure-code: test isa encode/decode with various object sizes

Create an encode_decode() helper method to be called from the
encode_decode test function with various object size arguments. The
helper method is a copy/paste of the previous test that was using a
single object of a fixed size. The test is slightly adapted to
accommodate for different object sizes but the logic is not modified.

The object sizes being tested are chosen to be under the size of the
required size alignment or on multiple pages, size aligned or not.

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