minor #17903 Validate XLIFF translation files (javiereguiluz)

This PR was squashed before being merged into the 2.3 branch (closes #17903).

Discussion
----------

Validate XLIFF translation files

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

In #17902 @stof proposed to add a simple test to validate `.xlf` files (to avoid issues like #17893).

This is a proposal for that test. My questions:

**1)** Do you agree displaying detailed error messages when the XML is not valid. Example output for the #17893 error:

![error_log](https://cloud.githubusercontent.com/assets/73419/13250664/932d2f14-da2b-11e5-8ea0-ecb43f58feea.png)

Or is it enough displaying just the PHPUnit error message and let the user figure out the details?

![simple_error](https://cloud.githubusercontent.com/assets/73419/13250671/a3b4bfd2-da2b-11e5-9277-454d0dd160b2.png)

**2)** How do I validate the translation files of the Security and Form components? Do I just copy+paste this test in those components?

Commits
-------

db03055 Validate XLIFF translation files

src/Symfony/Component/Form/Tests/Resources/TranslationFilesTest.php

@@ -0,0 +1,31 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Form\Tests\Resources;
+
+class TranslationFilesTest extends \PHPUnit_Framework_TestCase
+{
+    /**
+     * @dataProvider provideTranslationFiles
+     */
+    public function testTranslationFileIsValid($filePath)
+    {
+        \PHPUnit_Util_XML::loadfile($filePath, false, false, true);
+    }
+
+    public function provideTranslationFiles()
+    {
+        return array_map(
+            function ($filePath) { return (array) $filePath; },
+            glob(dirname(dirname(__DIR__)).'/Resources/translations/*.xlf')
+        );
+    }
+}

src/Symfony/Component/Security/Tests/Resources/TranslationFilesTest.php

@@ -0,0 +1,31 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Security\Tests\Resources;
+
+class TranslationFilesTest extends \PHPUnit_Framework_TestCase
+{
+    /**
+     * @dataProvider provideTranslationFiles
+     */
+    public function testTranslationFileIsValid($filePath)
+    {
+        \PHPUnit_Util_XML::loadfile($filePath, false, false, true);
+    }
+
+    public function provideTranslationFiles()
+    {
+        return array_map(
+            function ($filePath) { return (array) $filePath; },
+            glob(dirname(dirname(__DIR__)).'/Resources/translations/*.xlf')
+        );
+    }
+}

src/Symfony/Component/Validator/Tests/Resources/TranslationFilesTest.php

@@ -0,0 +1,31 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Validator\Tests\Resources;
+
+class TranslationFilesTest extends \PHPUnit_Framework_TestCase
+{
+    /**
+     * @dataProvider provideTranslationFiles
+     */
+    public function testTranslationFileIsValid($filePath)
+    {
+        \PHPUnit_Util_XML::loadfile($filePath, false, false, true);
+    }
+
+    public function provideTranslationFiles()
+    {
+        return array_map(
+            function ($filePath) { return (array) $filePath; },
+            glob(dirname(dirname(__DIR__)).'/Resources/translations/*.xlf')
+        );
+    }
+}