Filesystem: annotate the one network test with a "network" group.

Tests that require network access can be problematic, because they
depend on some external state not under your control. That can lead to
"random" failures when the code in question actually works fine. The
Filesystem component has one such test, and this commit adds it to the
"network" group (for PHPUnit).

Doing so lets the user skip that particular test, by running phpunit
with the --exclude-group flag. We take advantage of this in Gentoo,
where every user has the ability to run the test suite but network
access is forbidden.

src/Symfony/Component/Filesystem/Tests/FilesystemTest.php

@@ -156,6 +156,9 @@ class FilesystemTest extends FilesystemTestCase
         $this->assertEquals('SOURCE FILE', file_get_contents($targetFilePath));
     }
 
+    /**
+     * @group network
+     */
     public function testCopyForOriginUrlsAndExistingLocalFileDefaultsToCopy()
     {
         $sourceFilePath = 'http://symfony.com/images/common/logo/logo_symfony_header.png';