Add additional student name to Grade School test
Add additional student name to Grade School test

Without the third student name the test might pass without an
implementation (if the student names are prepended instead of appended
in the add implementation).

tjcelaya/xelixir,montague/xelixir,exercism/xelixir,elasticdog/xelixir,cezarykosko/xelixir,Egogre/xelixir,parkerl/xelixir,victorlcampos/xelixir,wobh/xelixir