reverted:
--- b/core/modules/serialization/tests/src/Functional/NormalizerTestBase.php
+++ a/core/modules/serialization/src/Tests/NormalizerTestBase.php
@@ -1,6 +1,6 @@
 <?php
 
+namespace Drupal\serialization\Tests;
-namespace Drupal\Tests\serialization\Functional;
 
 use Drupal\Tests\serialization\Kernel\NormalizerTestBase as SerializationNormalizerTestBase;
 
diff -u b/core/modules/serialization/tests/src/Functional/RegisterSerializationClassesCompilerPassTest.php b/core/modules/serialization/tests/src/Unit/RegisterSerializationClassesCompilerPassTest.php
--- b/core/modules/serialization/tests/src/Functional/RegisterSerializationClassesCompilerPassTest.php
+++ b/core/modules/serialization/tests/src/Unit/RegisterSerializationClassesCompilerPassTest.php
@@ -1,6 +1,6 @@
 <?php
 
-namespace Drupal\Tests\serialization\Functional;
+namespace Drupal\Tests\serialization\Unit;
 
 use Drupal\Core\DependencyInjection\ContainerBuilder;
 use Drupal\serialization\RegisterSerializationClassesCompilerPass;