Revamped argument parsing unit-tests (#12157)

* Updated the argument-parsing test suite to use a more declarative set of Option classes and included a new test for exhaustiveness which should assure that we are always testing every possible option available in the parser.

* Added a new test to check that all the expected options have default values.

* Converted default values to use the defaults module where appropriate to make the test suite more portable since the original hard-coded values were gathered from my development machine.

Author: Rostepher