minor #61140 [Config] Fix GeneratedConfigTest not being able to generate snapshots anymore (KevinVanSonsbeek)

This PR was merged into the 7.2 branch.

Discussion
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[Config] Fix GeneratedConfigTest not being able to generate snapshots anymore

| Q             | A
| ------------- | ---
| Branch?       | 7.2
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Issues        | Fix #61139
| License       | MIT

PR #57614 Removed multiple uses of `uniqid`. In the specific PR changes were made to the GeneratedConfigTest, to change how the directory was built for the config.

This inadvertedly broke the generating of snapshot files, due to the outputDir being changed to a by reference argument, and always overwriting the value to a temp dir. (While the snapshot files should not be written to a temp dir)

Commits
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8e8daf1cf97 bugfix(#61139): Only generate an outputDir if none is set.

Author: nicolas-grekas

Tests/Builder/GeneratedConfigTest.php

@@ -159,10 +159,12 @@ public function testSetExtraKeyMethodIsNotGeneratedWhenAllowExtraKeysIsFalse()
      */
     private function generateConfigBuilder(string $configurationClass, ?string &$outputDir = null)
     {
-        $outputDir = tempnam(sys_get_temp_dir(), 'sf_config_builder_');
-        unlink($outputDir);
-        mkdir($outputDir);
-        $this->tempDir[] = $outputDir;
+        if (null === $outputDir) {
+            $outputDir = tempnam(sys_get_temp_dir(), 'sf_config_builder_');
+            unlink($outputDir);
+            mkdir($outputDir);
+            $this->tempDir[] = $outputDir;
+        }
 
         $configuration = new $configurationClass();
         $rootNode = $configuration->getConfigTreeBuilder()->buildTree();