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.
Merge pull request #905 from drueck/elixir-grade-school-lists

Change grade school tests and example to encourage idiomatic Elixir list building