tests: Add PHPUnit tests for methods in MagicWordFactory::class

tests: Add PHPUnit tests for methods in MagicWordFactory::class

* testGetContentLanguage() - covers the getContentLanguage() method.
* testGet() - covers the get() method in the MagicWordFactory class.
* testGetVariableIDs() - covers the getVariableIDs method.
* testGetSubstIDs() - covers the getSubstIDs() method.
* testGetCacheTTL() - covers the getCacheTTL() method. This covers
  both paths when there is a valid and an invalid caching hint.
* makeMagicWordFactory() - a helper function for creating an object
  of the MagicWordFactory class.
* testGetDoubleUnderscoreArray() - covers getDoubleUnderscoreArray()
  method and also calls newArray() under the hood so that is also
  covered.

Change-Id: I5d2166f155e31900cb40c22fb976e81d0d545627