Allow running PHPUnit with "xdebug.scream" ON

Since #25733 the Kernel attempts to unlink the legacy container while being built.
This throws an error if the file did not exist, for example on a clean install, on the build, which is then silenced.

That's fine on production systems, but on our build we have enabled "xdebug.scream" in order to visualise every errors, which basically un-silences the errors. I believe there should not be a need to silence anything on a usual, clean usage of the system.
Making this `unlink` conditional fixes it.

Could you please approve and merge this PR?
Thanks

src/Symfony/Component/HttpKernel/Kernel.php

@@ -862,7 +862,10 @@ protected function dumpContainer(ConfigCache $cache, ContainerBuilder $container
             $fs->dumpFile($dir.$file, $code);
             @chmod($dir.$file, 0666 & ~umask());
         }
-        @unlink(\dirname($dir.$file).'.legacy');
+        $legacyFile = \dirname($dir.$file).'.legacy';
+        if (file_exists($legacyFile)) {
+            @unlink($legacyFile);
+        }
 
         $cache->write($rootCode, $container->getResources());
     }