function testFirstInternalNode() {
  var tree = insertToTree(5);
  var expected =
      '2[13]\n' +
      '_{0|1}_\n' +
      '0[5|9]  1[13|17|21]\n' +
      '_{5/9}2  0{13/17/21}2\n';
  assertEquals(expected, tree.toString());
}
Splits the root node to form new root node.

9|13|17|21

insert 5

    13
   /  \
 5|9  13|17|21