PgTap, unit test a foreign key is deferrable
Ok, it appears there are no methods to test such thing with PgTap for now so I ended up with

SELECT results_eq(
    $$select condeferrable, condeferred from pg_catalog.pg_constraint where conname = 'default_article_id_fk'$$,
    $$values ('t'::bool, 'f'::bool)$$,
    '"default_article_id_fk" is deferrable initially immediate.'
);


That does the job.