Add pending tests to match phone_number doc strings
Add pending tests to match phone_number doc strings

#### Context

While completing http://exercism.io/tracks/elixir/exercises/phone-number , I was
surprised to see all the tests pass when I'd not written code to satisfy the doc
strings at
https://github.com/exercism/elixir/blob/500bf38d7113c33d0051a73d1732b51bc829865d/exercises/phone-number/phone_number.exs#L43-L47

#### Change

 - Add pending tests to match phone_number doc strings

exercism/xelixir,parkerl/xelixir