add test to cover #search on InternalNode, see note

This is a really bad test that shouldn't live very long. It's complicated
primarily because it doesn't account for the fact that all the elements of
the array of children will be either InternalNodes or Leafs, the latter of
which act as the stop case. Instead it just asserts that the 'right' thing
is called, but it should be enough to let us get on with the work of
implementing #search, which should make it easier to implement #split