eo: constructor tests do not use eo_add_custom(..)

eo: constructor tests do not use eo_add_custom(..)

as stated in f92e5d50, instead of using eo_add_custom() thus a custom
constructor and maybe overriding the default constructor to block it,

   - use the default constructor to build the object
   - add calls to eo_add(), to initialize the object
     eo_add(class, parent, val_a_set(1), val_b_set(2), ... );
   - override eo_finalize to validate the object and if needed,
     use eo_error_set(obj) to abort object construction