Try another loading hack

Author: mglaman

src/Drupal/DrupalAutoloader.php

@@ -104,8 +104,10 @@ public function register(Container $container): void
 
         // class_alias is not supported by OptimizedDirectorySourceLocator or AutoloadSourceLocator,
         // so we manually load this PHPUnit compatibility trait that exists in Drupal 8.
-        if (file_exists($this->drupalRoot . '/core/tests/Drupal/Tests/PhpunitCompatibilityTrait.php')) {
-            require_once $this->drupalRoot . '/core/tests/Drupal/Tests/PhpunitCompatibilityTrait.php';
+        $phpunitCompatTraitFilepath = $this->drupalRoot . '/core/tests/Drupal/Tests/PhpunitCompatibilityTrait.php';
+        if (file_exists($phpunitCompatTraitFilepath)) {
+            require_once $phpunitCompatTraitFilepath;
+            $this->autoloader->addClassMap(['Drupal\\Tests\\PhpunitCompatibilityTrait' => $phpunitCompatTraitFilepath]);
         }
 
         foreach ($this->moduleData as $extension) {