Issue #3116856 by alexpott, Mile23: Workaround PHPUnit 8 warnings

core/phpunit.xml.dist

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

core/tests/Drupal/KernelTests/KernelTestBase.php

core/tests/Drupal/Tests/BrowserTestBase.php

...pal/Tests/Component/Annotation/Doctrine/DocParserTest.php

.../Drupal/Tests/Component/FileSecurity/FileSecurityTest.php

...rupal/Tests/Composer/Plugin/Scaffold/AssertUtilsTrait.php

...sts/Composer/Plugin/Scaffold/Integration/AppendOpTest.php

...ts/Composer/Plugin/Scaffold/Integration/ReplaceOpTest.php

...Tests/Composer/Plugin/Scaffold/Integration/SkipOpTest.php

...upal/Tests/Composer/Plugin/VendorHardening/ConfigTest.php

core/tests/Drupal/Tests/Core/Command/QuickStartTest.php

...tests/Drupal/Tests/Listeners/DeprecationListenerTrait.php

core/tests/Drupal/Tests/Traits/PHPUnit8Warnings.php

core/tests/Drupal/Tests/UnitTestCase.php