Improve PHPUnit code coverage

Improve PHPUnit code coverage

XmlJs:
 - simple class == simple test

GlobalFunctions:
 - wfArrayDiff2: simple test, please add more

XmlSelectTest:
 - test for constructor
 - tests for setDefault. Found a potential bug when setting a default after
   options have been added. The default option will not get marked as selected!
   We should change our code to generates HTML for options at rendering time
   or throw an exception that default can not be used if an option is present.

MediaWiki:
 - basic placeholder generated by PHPUnit
 - tests for setVal() / getVal()