phpunit: Avoid use of deprecated getMock for PHPUnit 5 compat

The default will remain PHPUnit 4.x due to PHP 5.5 support.

But, we should allow developers to run tests with newer PHPUnit
versions which are noticably faster (especially for code coverage
reports).

* <https://github.com/sebastianbergmann/phpunit/wiki/Release-Announcement-for-PHPUnit-5.4.0>
  PHPUnit 5 deprecates the getMock() shortcut for getMockBuilder()->getMock().
  It instead introduces the shortcut createMock() which has better defaults
  than getMockBuilder(). For example, it sets 'disableArgumentCloning' and
  other things by default.

  Going forward, code should either use getMockBuilder directly and configure
  it using the setter methods (instead of the confusing variadic arguments
  of getMock) or simply use the new minimalistic createMock method. This patch
  backports the createMock method to MediaWikiTestCase so that we can start
  using it.

Change-Id: I091c0289b21d2b1c876adba89529dc3e72b99af2