Answer by kglr for bind specific 'Slots' to specific 'Map'

You can do it even without slots:

Map[Map[Flatten @* List] @* FoldList[List]] @ test  

 {{{4}, {4, 2}, {4, 2, 2}},   {{9}, {9, 1}, {9, 1, 5}},   {{5}, {5, 2}, {5,  2, 9}, {5, 2, 9, 3}},   {{5}, {5, 2}, {5, 2, 7}, {5, 2, 7, 1}, {5, 2, 7, 1, 1}}}

Also

Map[Extract[#, Map[List] @ Range @ Range @ Length @ #] &] @ test  

 {{{4}, {4, 2}, {4, 2, 2}},   {{9}, {9, 1}, {9, 1, 5}},   {{5}, {5, 2}, {5,  2, 9}, {5, 2, 9, 3}},   {{5}, {5, 2}, {5, 2, 7}, {5, 2, 7, 1}, {5, 2, 7, 1, 1}}}