Use writeError for xdebug warning and fix running test suite with xdebug

src/Composer/Console/Application.php

@@ -112,7 +112,7 @@ class Application extends BaseApplication
         }
 
         if (extension_loaded('xdebug')) {
-            $io->write('<warning>You are running composer with xdebug enabled. This has a major impact on runtime performance. See https://getcomposer.org/xdebug</warning>');
+            $io->writeError('<warning>You are running composer with xdebug enabled. This has a major impact on runtime performance. See https://getcomposer.org/xdebug</warning>');
         }
 
         if (defined('COMPOSER_DEV_WARNING_TIME')) {

tests/Composer/Test/ApplicationTest.php

@@ -28,7 +28,14 @@ class ApplicationTest extends TestCase
             ->method('getFirstArgument')
             ->will($this->returnValue('list'));
 
-        $outputMock->expects($this->once())
+        $index = 0;
+        if (extension_loaded('xdebug')) {
+            $outputMock->expects($this->at($index++))
+                ->method("write")
+                ->with($this->equalTo('<warning>You are running composer with xdebug enabled. This has a major impact on runtime performance. See https://getcomposer.org/xdebug</warning>'));
+        }
+
+        $outputMock->expects($this->at($index++))
             ->method("write")
             ->with($this->equalTo(sprintf('<warning>Warning: This development build of composer is over 60 days old. It is recommended to update it by running "%s self-update" to get the latest version.</warning>', $_SERVER['PHP_SELF'])));