8 files declare their use of PhpUnitCompatibilityTrait in Drupal 9

AnnotationBridgeDecoratorTest.php in core/tests/Drupal/Tests/Component/Annotation/Plugin/Discovery/AnnotationBridgeDecoratorTest.php

BuildTestBase.php in core/tests/Drupal/BuildTests/Framework/BuildTestBase.php

ContainerTest.php in core/tests/Drupal/Tests/Component/DependencyInjection/ContainerTest.php

Contains \Drupal\Tests\Component\DependencyInjection\ContainerTest.

KernelTestBase.php in core/tests/Drupal/KernelTests/KernelTestBase.php

OptimizedPhpArrayDumperTest.php in core/tests/Drupal/Tests/Component/DependencyInjection/Dumper/OptimizedPhpArrayDumperTest.php

Contains \Drupal\Tests\Component\DependencyInjection\Dumper\OptimizedPhpArrayDumperTest.

PluginManagerBaseTest.php in core/tests/Drupal/Tests/Component/Plugin/PluginManagerBaseTest.php

PoStreamWriterTest.php in core/tests/Drupal/Tests/Component/Gettext/PoStreamWriterTest.php

VendorHardeningPluginTest.php in core/tests/Drupal/Tests/Composer/Plugin/VendorHardening/VendorHardeningPluginTest.php