Add phpunit configuration file

Use the configuration file to pass the location of the tests and
autoload class files.

Pass -sp to phpcs command

tests/php/LanguageUtilTest.php

@@ -1,6 +1,4 @@
 <?php
-require __DIR__ . '/../../src/LanguageUtil.php';
-
 use PHPUnit\Framework\TestCase;
 use Wikimedia\LanguageData\LanguageUtil;