Your use of case
is fine, but have a look where you cons the new value on the list, and where the solution does.
testFunc = const Nothing
unfold testFunc 1 == [] -- your version prepends only if f a isn't Nothing
unfold' testFunc 1 == [1] -- the solution _always_ prepends the current value
Also, you're using the same value all the time.
unfold :: (a -> Maybe a) ->a -> [a]
unfold f a = a : case f a of -- cons before the case
Just n -> unfold f n -- use n as parameter for f
Nothing -> []