Use PHPUnit pre-conditions

See https://phpunit.de/manual/current/en/incomplete-and-skipped-tests.html#incomplete-and-skipped-tests.requires.tables.api

tests/Composer/Test/Util/FilesystemTest.php

@@ -179,6 +179,7 @@ class FilesystemTest extends TestCase
 
     /**
      * @link https://github.com/composer/composer/issues/3157
+     * @requires function symlink
      */
     public function testUnlinkSymlinkedDirectory()
     {
@@ -207,6 +208,7 @@ class FilesystemTest extends TestCase
 
     /**
      * @link https://github.com/composer/composer/issues/3144
+     * @requires function symlink
      */
     public function testRemoveSymlinkedDirectoryWithTrailingSlash()
     {
@@ -239,11 +241,4 @@ class FilesystemTest extends TestCase
         $this->assertFalse(file_exists($symlinkedTrailingSlash));
         $this->assertFalse(file_exists($symlinked));
     }
-
-    private function skipTestIfSymlinkPhpFunctionIsMissing()
-    {
-        if (!function_exists('symlink')) {
-            $this->markTestSkipped('The php symlink() function for symbolic links is not available on this platform');
-        }
-    }
 }