6 files declare their use of StubPDO in Drupal 9

ConditionTest.php in core/tests/Drupal/Tests/Core/Database/ConditionTest.php

ConnectionTest.php in core/tests/Drupal/Tests/Core/Database/ConnectionTest.php

ConnectionTest.php in core/tests/Drupal/Tests/Core/Database/Driver/sqlite/ConnectionTest.php

LogTest.php in core/tests/Drupal/Tests/Core/Database/LogTest.php

OrderByTest.php in core/tests/Drupal/Tests/Core/Database/OrderByTest.php

SelectExtenderTest.php in core/tests/Drupal/KernelTests/Core/Database/SelectExtenderTest.php