Add unit test for serialisation of DiskLogicalId and Nodes

Since the DiskLogicalId type is manually serialised/deserialised (see
Objects.hs, `encodeDLid' and `decodeDLId'), let's add a test that
checks that these are idempotent when combined.

Since we're at it, let's add the same test for Node serialisation,
which already has an Arbitrary instance.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>