minor #15338 [Security/Http] Fix test relying on a private property (nicolas-grekas)

This PR was merged into the 2.6 branch.

Discussion
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[Security/Http] Fix test relying on a private property

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

I'm not 100% sure what was tested by this, but this test was using a private property (`AnonymousToken->key`), that has been renamed to `secret` in later Sf versions.

Commits
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2d29ac1 [Security/Http] Fix test relying on a private property

src/Symfony/Component/Security/Http/Tests/Firewall/AnonymousAuthenticationListenerTest.php

@@ -54,10 +54,9 @@ class AnonymousAuthenticationListenerTest extends \PHPUnit_Framework_TestCase
         $authenticationManager
             ->expects($this->once())
             ->method('authenticate')
-            ->with(self::logicalAnd(
-                       $this->isInstanceOf('Symfony\Component\Security\Core\Authentication\Token\AnonymousToken'),
-                       $this->attributeEqualTo('key', 'TheKey')
-            ))
+            ->with($this->callback(function ($token) {
+                return 'TheKey' === $token->getKey();
+            }))
             ->will($this->returnValue($anonymousToken))
         ;