It is called "test", so I believe it is different with "casting" or "recasting" a spell.
What is the point of recasting a spell? It's the same as casting it once again. So what's the point of casting it before any fight If I, as you say, have to cast it once again when I want to go in hth?
In my opinion the spell "Hammer of Sigmar" is still active in the subsequent rounds, because ending time isn't specified like in other spells. E.g. Armour of Righteousness has a specified time when the spell ends, Shield of Faith also. And according to FAQ "The spell lasts until the model that casted it is taken Out Of Action". So I think I have to only check (test) if I can use it this turn. Failed test doesn't end my spell but only "deactivate" it the turn when test was failed, and I can make another test in next rounds, but not casting a new "Hammer of Sigmar". So according to my words there should be possibility to test a "Hamer of Sigmar" and cast a spell. Is my way of thinking is correct? I just take some rules and combined them together. :-)