minor #32723 [Messenger] fix test (xabbuh)

This PR was merged into the 4.3 branch.

Discussion
----------

[Messenger] fix test

| Q             | A
| ------------- | ---
| Branch?       | 4.3
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

fixes a test that was added in the `4.3` branch now that #32700 is merged up

Commits
-------

16fc81f fix test

src/Symfony/Component/Messenger/Tests/TraceableMessageBusTest.php

@@ -37,7 +37,7 @@ public function testItTracesDispatch()
         unset($actualTracedMessage['callTime']); // don't check, too variable
         $this->assertEquals([
             'message' => $message,
-            'stamps' => [[$stamp]],
+            'stamps' => [$stamp],
             'caller' => [
                 'name' => 'TraceableMessageBusTest.php',
                 'file' => __FILE__,