minor #20785 [VarDumper] fix tests when xdebug is enabled (ShinDarth)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #20785).

Discussion
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[VarDumper] fix tests when xdebug is enabled

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #20778
| License       | MIT

- Disabled some test cases when xdebug is enabled, see https://github.com/symfony/symfony/issues/20778

Commits
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488ebbf [VarDumper] fix tests when xdebug is enabled

src/Symfony/Component/VarDumper/Tests/VarClonerTest.php

@@ -137,6 +137,10 @@ EOTXT;
 
     public function testJsonCast()
     {
+        if (ini_get('xdebug.overload_var_dump') == 2) {
+            $this->markTestSkipped('xdebug is active');
+        }
+
         $data = (array) json_decode('{"1":{}}');
 
         $cloner = new VarCloner();