minor #36568 [PhpUnitBridge] Improve dirname usage (Jean85)

This PR was merged into the 3.4 branch.

Discussion
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[PhpUnitBridge] Improve dirname usage

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | Fix #36499
| License       | MIT

This is a very small performance improvement on top of #36539

Commits
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e721cfd Improve dirname usage

src/Symfony/Bridge/PhpUnit/Legacy/SymfonyTestsListenerTrait.php

@@ -49,7 +49,7 @@ public function __construct(array $mockedNamespaces = array())
             \PHPUnit_Util_Blacklist::$blacklistedClassNames[__CLASS__] = 2;
         } elseif (method_exists(Blacklist::class, 'addDirectory')) {
             (new BlackList())->getBlacklistedDirectories();
-            Blacklist::addDirectory(\dirname(\dirname((new \ReflectionClass(__CLASS__))->getFileName())));
+            Blacklist::addDirectory(\dirname((new \ReflectionClass(__CLASS__))->getFileName(), 2));
         } else {
             Blacklist::$blacklistedClassNames[__CLASS__] = 2;
         }