minor #2870 [Translator][Tests] Minor fixes in tests context (Kocal)

This PR was merged into the 2.x branch.

Discussion
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[Translator][Tests] Minor fixes in tests context

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Docs?         | no <!-- required for new features -->
| Issues        | Fix #... <!-- prefix each issue number with "Fix #", no need to create an issue if none exist, explain below instead -->
| License       | MIT

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Cherry-picked from #2867, the following fixes only apply inside tests context, it does not have impact in real-life scenarios:
1. The static variable `$alreadyGenerated` now becomes a private property `alreadyGeneratedConstants. When tests were executed randomly in #2867, it was impossible to assert on generated translations files because constants names were kept in static cache even when the class `TranslationsDumper` was initialized for each test.
2. `self::$cacheDir` is a directory, so using `rmdir` is the best way to go instead of `unlink`

Commits
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6e58d54 [Translator] Move already generated constants from static var to class property

Author: Kocal

src/Translator/.gitignore

@@ -3,3 +3,5 @@
 /composer.lock
 /phpunit.xml
 /.phpunit.result.cache
+
+/var

src/Translator/src/TranslationsDumper.php

@@ -34,6 +34,7 @@ class TranslationsDumper
 {
     private array $excludedDomains = [];
     private array $includedDomains = [];
+    private array $alreadyGeneratedConstants = [];
 
     public function __construct(
         private string $dumpDir,
@@ -184,16 +185,14 @@ private function getLocaleFallbacks(MessageCatalogueInterface ...$catalogues): a
 
     private function generateConstantName(string $translationId): string
     {
-        static $alreadyGenerated = [];
-
         $translationId = s($translationId)->ascii()->snake()->upper()->replaceMatches('/^(\d)/', '_$1')->toString();
         $prefix = 0;
         do {
             $constantName = $translationId.($prefix > 0 ? '_'.$prefix : '');
             ++$prefix;
-        } while ($alreadyGenerated[$constantName] ?? false);
+        } while ($this->alreadyGeneratedConstants[$constantName] ?? false);
 
-        $alreadyGenerated[$constantName] = true;
+        $this->alreadyGeneratedConstants[$constantName] = true;
 
         return $constantName;
     }

src/Translator/tests/CacheWarmer/TranslationsCacheWarmerTest.php

@@ -28,7 +28,7 @@ public static function setUpBeforeClass(): void
 
     public static function tearDownAfterClass(): void
     {
-        @unlink(self::$cacheDir);
+        @rmdir(self::$cacheDir);
     }
 
     public function test()