Fix 'Double callback' warning in get hierarchy tests

[chances]

i.e.

  hierarchy
    GET /hierarchy
      ✓ should render an empty item hierarchy tree
double callback!
      ✓ should render an item hierarchy tree with one item with no children
double callback!
      ✓ should render an item hierarchy with one rooted item containing a single child
double callback!
      ✓ should render an item heirarchy with one rooted item containing multiple children
double callback!
      ✓ should render an item hierarchy with one rooted item containing a nested child
double callback!
      ✓ should render an item hierarchy with multiple rooted items with no children
double callback!
      ✓ should render an item hierarchy with multiple rooted items with no children
double callback!
      ✓ should render an item hierarchy with multiple rooted items with children
double callback!
      ✓ should render an item hierarchy with multiple rooted items with nested items
double callback!