minor #18576 [Cache] Fix test tearDown (nicolas-grekas)

This PR was merged into the 3.1-dev branch.

Discussion
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[Cache] Fix test tearDown

| Q             | A
| ------------- | ---
| Branch?       | 3.1
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
-------

9ca22e9 [Cache] Fix test tearDown

src/Symfony/Component/Cache/Tests/Adapter/FilesystemAdapterTest.php

@@ -35,6 +35,9 @@ public static function tearDownAfterClass()
 
     public static function rmdir($dir)
     {
+        if (!file_exists($dir)) {
+            return;
+        }
         if (!$dir || 0 !== strpos(dirname($dir), sys_get_temp_dir())) {
             throw new \Exception(__METHOD__."() operates only on subdirs of system's temp dir");
         }