ErasureCodeJerasure: unit test common to all techniques

ErasureCodeJerasure: unit test common to all techniques

A typed unit test is defined and must run regardless of the technique.
When a new technique is derived from ErasureCodeJerasure, it is added
to the JerasureTypes typedef and the test will validate that:

  * it provides reasonable defaults for the technique specific
    parameters
  * it modifies the k, m and w to reasonable defaults depending
    on the imposed constraints ( for instance Liber8tion requires
    that w == 8 but the test sets it to 7 )
  * the encoding of K=2, M=2 produces 4 chunks, the first two
    of which contains the original buffer data showing the
    code is systematic
  * decoding when all 4 chunks are available indeed retrieves
    the original buffer content
  * decoding when the two data chunks are are missing indeed
    retrieves the original buffer content

https://github.com/dachary/ceph/tree/wip-5879 refs #5879

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