Change grade-school tests/example to encourage iodimatic Elixir

@pminten reminded me that in Elixir lists are implemented as singly
linked lists and as a result it is O(1) to add an item to the beginning
of a list, while it is O(n) to add an item to the end of a list.
Previously the tests were expecting students to be added to the end of
the list, which forces the less efficient implementation.

I updated the tests so that they do not care in what order the
students are added to the list, and updated the example to use the
`[ head | tail ]` form of adding an item to the head of a list.

Hopefully allowing freedom in what order the students can be added to
the list will encourage conversation from nitpickers as to the optimal
and idiomatic way to do this.