Step 2

* Added test testAddThreeNumbers - Tests that passing 3 numbers returns
their sum. that validates "1,2,3" returns 6.
* Fixed failing test by amending "getInt(bothNumbers[0]) +
getInt(bothNumbers[1]);" to iterate the array adding to a total.
* Refactored now that the numbers.contains(",") is now redundant.
* Considered adding a test for 3 numbers the same but feel that it is
not required.
* Feeling a bit worried that the input isn't being validated at all but
continuing none the less, a NumberFormatException will protect us for
now.