make clashes less likely and use silencer

tests/Composer/TestCase.php

@@ -16,6 +16,7 @@ use Composer\Semver\VersionParser;
 use Composer\Package\AliasPackage;
 use Composer\Semver\Constraint\Constraint;
 use Composer\Util\Filesystem;
+use Composer\Util\Silencer;
 
 abstract class TestCase extends \PHPUnit_Framework_TestCase
 {
@@ -62,8 +63,9 @@ abstract class TestCase extends \PHPUnit_Framework_TestCase
         $root = sys_get_temp_dir();
 
         do {
-            $unique = $root . DIRECTORY_SEPARATOR . uniqid('composer-test-');
-            if (!file_exists($unique) && false !== @mkdir($unique, 0777)) {
+            $unique = $root . DIRECTORY_SEPARATOR . uniqid('composer-test-' . rand(1000, 9000));
+
+            if (!file_exists($unique) && Silencer::call('mkdir', $unique, 0777)) {
                 return realpath($unique);
             }
         } while (--$attempts);