minor #41567 [Security] Fix SerializableUser fixture (derrabus)

This PR was merged into the 4.4 branch.

Discussion
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[Security] Fix SerializableUser fixture

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | N/A
| License       | MIT
| Doc PR        | N/A

That's hopefully the last `Serializable` implementation we need to fix. 😓

Commits
-------

8fbf3bb3c7 [Security] Fix SerializableUser fixture

src/Symfony/Component/Security/Core/Tests/Authentication/Token/AbstractTokenTest.php

@@ -322,12 +322,22 @@ class SerializableUser implements UserInterface, \Serializable
 
     public function serialize(): string
     {
-        return serialize($this->name);
+        return serialize($this->__serialize());
     }
 
     public function unserialize($serialized): void
     {
-        $this->name = unserialize($serialized);
+        $this->__unserialize(unserialize($serialized));
+    }
+
+    public function __serialize(): array
+    {
+        return ['name' => $this->name];
+    }
+
+    public function __unserialize(array $data): void
+    {
+        ['name' => $this->name] = $data;
     }
 }