Do not use the test class to test the Callback rule

That will help us later because, on PHPUnit 10, all data providers need
to be static.

Signed-off-by: Henrique Moody <henriquemoody@gmail.com>

tests/unit/Rules/CallbackTest.php

@@ -32,7 +32,7 @@ final class CallbackTest extends RuleTestCase
     {
         return [
             [new Callback('is_a', 'stdClass'), new stdClass()],
-            [new Callback([$this, 'sampleCallbackUsedInsideThisTest']), 'test'],
+            [new Callback([new AlwaysValid(), 'validate']), 'test'],
             [new Callback('is_string'), 'test'],
             [
                 new Callback(static function () {
@@ -43,11 +43,6 @@ final class CallbackTest extends RuleTestCase
         ];
     }
 
-    public function sampleCallbackUsedInsideThisTest(): bool
-    {
-        return true;
-    }
-
     /**
      * {@inheritDoc}
      */