Use PHPunit DOM assertions instead of deprecations

As of PHPUnit 4.2 assertions built-in into PHPUnit about the DOM
are extracted and refactored in https://github.com/phpunit/phpunit-dom-assertions

I've replaced `PHPUnit_Framework_TestCase` with `PHPUnit_Framework_DOMTestCase`.

The one failing test with the new assertions was `Jam_Form_GeneralTest::test_file`.
`[value=""]` no longer matches when the `value` attribute is missing altogether.