Issue #2542486 by neclimdul, Wim Leers, Mile23: Fix risky phpunit tests

core/modules/simpletest/tests/src/Unit/WebTestBaseTest.php

core/tests/Drupal/Tests/Component/DrupalComponentTest.php

...al/Tests/Component/PhpStorage/FileStorageReadOnlyTest.php

...sts/Drupal/Tests/Component/PhpStorage/FileStorageTest.php

...ts/Component/PhpStorage/MTimeProtectedFileStorageBase.php

.../Drupal/Tests/Component/PhpStorage/PhpStorageTestBase.php

core/tests/Drupal/Tests/ComposerIntegrationTest.php

core/tests/Drupal/Tests/Core/Render/RendererBubblingTest.php

...sts/Drupal/Tests/Core/Render/RendererPlaceholdersTest.php

core/tests/Drupal/Tests/Core/Render/RendererTestBase.php