Move some providers in new MediaWikiProvide class

Move some providers in new MediaWikiProvide class

The MediaWikiProvide class would host providing methods reused in different
tests classes.  To use it, just use the Class::function syntax.

Example:
  /** @dataProvider MediaWikiProvide::Months */

Made a svn copy of phpunit/includes/parser/MagicVariableTest.php
to save the history.