Add more tests for CQL external functions

[Darren Duncan]

This is a follow-up to issue #52.

The CQL external functions feature deserves a more thorough set of unit tests.

An example of such is one that tests a possible use case like libraries that reference a function with the same signature, but have different results, like the following (written by @c-schuler):

library CqlExternalFunctionsTest version '3'

include AnotherLibrary version '1' called AnotherLibrary

context Patient

define function MyStringOperation(x String) returns String : external

define CallCurrentStringOperation: 
    MyStringOperation('Testing') // returns TESTING

define CallAnotherStringOperation: 
    AnotherLibrary.MyStringOperation('Testing') // returns testing

But more than the above should be added so the additions are not arbitrary as a sum.