Although some sources suggest ‘I am going to …’ rather than ‘I shall …’ emphasises intent or immediacy/near future, in practice in spoken English there is no difference. (So I don’t think the test on the simple future should mark it as a mistake.) Perhaps, as in French, the ‘going to’ version of the future tense is more common. In English this might be because many people do not know the distinction between shall and will, especially as both are abbreviated to « ‘ll », and I have not found that distinction in Kwiziq yet (i.e. shall is for first person, will is for 2nd and 3rd persons, singular and plural: I shall but you will.)