11 files declare their use of PhpUnitWarnings in Drupal 9

AppendOpTest.php in core/tests/Drupal/Tests/Composer/Plugin/Scaffold/Integration/AppendOpTest.php

AssertUtilsTrait.php in core/tests/Drupal/Tests/Composer/Plugin/Scaffold/AssertUtilsTrait.php

BrowserTestBase.php in core/tests/Drupal/Tests/BrowserTestBase.php

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

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

FileStorageTest.php in core/tests/Drupal/Tests/Component/PhpStorage/FileStorageTest.php

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

ReplaceOpTest.php in core/tests/Drupal/Tests/Composer/Plugin/Scaffold/Integration/ReplaceOpTest.php

SkipOpTest.php in core/tests/Drupal/Tests/Composer/Plugin/Scaffold/Integration/SkipOpTest.php

UnitTestCase.php in core/tests/Drupal/Tests/UnitTestCase.php

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