bug symfony#25230 Use a more specific file for detecting the bridge (greg0ire)

This PR was merged into the 3.4 branch.

Discussion
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Use a more specific file for detecting the bridge

| Q             | A
| ------------- | ---
| Branch?       | 3.4 because this patch cannot apply before
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony#25228
| License       | MIT
| Doc PR        | n/a

Commits
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355bbd1 Use a more specific file for detecting the bridge

Author: nicolas-grekas

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;
     }