minor #31933 [DependencyInjection] fix the ValidateEnvPlaceHolderPassTest that was using a deprecated path for TreeBuilder (Simperfit)

This PR was merged into the 4.2 branch.

Discussion
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[DependencyInjection] fix the ValidateEnvPlaceHolderPassTest that was using a deprecated path for TreeBuilder

| Q             | A
| ------------- | ---
| Branch?       | 4.2
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | none   <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | none <!-- required for new features -->

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 - Bug fixes must be submitted against the lowest maintained branch where they apply
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This should be fixed in 4.2 because the deprecation has been introduced in 4.2 as said by @stof in https://github.com/symfony/symfony/pull/31932/files#r291600094

Commits
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caabd92f89 [DependencyInjection] fix the ValidateEnvPlaceHolderPassTest that was using a deprecated path for TreeBuilder

src/Symfony/Component/DependencyInjection/Tests/Compiler/ValidateEnvPlaceholdersPassTest.php

@@ -332,7 +332,7 @@ class EnvConfigurationWithoutRootNode implements ConfigurationInterface
 {
     public function getConfigTreeBuilder()
     {
-        return new TreeBuilder();
+        return new TreeBuilder('env_extension');
     }
 }
 
@@ -340,8 +340,8 @@ class ConfigurationWithArrayNodeRequiringOneElement implements ConfigurationInte
 {
     public function getConfigTreeBuilder()
     {
-        $treeBuilder = new TreeBuilder();
-        $treeBuilder->root('env_extension')
+        $treeBuilder = new TreeBuilder('env_extension');
+        $treeBuilder->getRootNode()
             ->children()
                 ->arrayNode('nodes')
                     ->isRequired()