Merge branch '3.4' into 4.3

* 3.4:
  [Yaml] fix test for PHP 7.4
  Add polyfill for TestCase::createMock()
  Skip tests that fatal-error on PHP 7.4 because of missing parent classes

Author: nicolas-grekas

Tests/CacheWarmer/ValidatorCacheWarmerTest.php

@@ -11,6 +11,7 @@
 
 namespace Symfony\Bundle\FrameworkBundle\Tests\CacheWarmer;
 
+use PHPUnit\Framework\Warning;
 use Symfony\Bridge\PhpUnit\ForwardCompatTestTrait;
 use Symfony\Bundle\FrameworkBundle\CacheWarmer\ValidatorCacheWarmer;
 use Symfony\Bundle\FrameworkBundle\Tests\TestCase;
@@ -25,6 +26,10 @@ class ValidatorCacheWarmerTest extends TestCase
 
     public function testWarmUp()
     {
+        if (\PHP_VERSION_ID >= 70400) {
+            throw new Warning('PHP 7.4 breaks this test, see https://bugs.php.net/78351.');
+        }
+
         $validatorBuilder = new ValidatorBuilder();
         $validatorBuilder->addXmlMapping(__DIR__.'/../Fixtures/Validation/Resources/person.xml');
         $validatorBuilder->addYamlMapping(__DIR__.'/../Fixtures/Validation/Resources/author.yml');