Use a more specific file for detecting the bridge

When using bin as a Composer bin-dir, this will break because both
composer.json and bin/simple-phpunit will exist relatively to the
current directory. This change checks for a non-binary file specific to
the bridge that will not have this issue.
Fixes #25228

src/Symfony/Bridge/PhpUnit/bin/simple-phpunit

@@ -27,7 +27,7 @@ if (PHP_VERSION_ID >= 70200) {
 }
 
 $root = __DIR__;
-while (!file_exists($root.'/composer.json') || file_exists($root.'/bin/simple-phpunit')) {
+while (!file_exists($root.'/composer.json') || file_exists($root.'/DeprecationErrorHandler.php')) {
     if ($root === dirname($root)) {
         break;
     }