message: improve BinaryTreeArray unit test
  make sure we visit children before the parents.
diff: @@ -24,16 +24,19 @@ TEST(BinaryTreeArray, basic) {
  struct Node {
  - size_t index;
  - size_t next;
  + size_t index = 0;
  + size_t next = 0;
  + bool visited = 0;
  };
  Node array[256];
  BinaryTreeArray::traverse(1,
  [&array](size_t index, size_t col, size_t next) {
  - array[index] = Node{ index, next };
  + array[index] = Node{ index, next, true };
  },
  [&array](size_t index, size_t l, size_t r, size_t next) {
  + EXPECT_TRUE(array[l].visited);
  + EXPECT_TRUE(array[r].visited);
  array[index] = Node{ index, next };
  });
  