Issue #1376122 by sun: Stream wrappers of parent site are leaking into all tests.

core/includes/file.inc

core/modules/file/lib/Drupal/file/Tests/CopyTest.php

core/modules/file/lib/Drupal/file/Tests/DeleteTest.php

core/modules/file/lib/Drupal/file/Tests/DownloadTest.php

...odules/file/lib/Drupal/file/Tests/FileManagedTestBase.php

core/modules/file/lib/Drupal/file/Tests/LoadTest.php

core/modules/file/lib/Drupal/file/Tests/MoveTest.php

...s/file/lib/Drupal/file/Tests/RemoteFileSaveUploadTest.php

core/modules/file/lib/Drupal/file/Tests/SaveDataTest.php

core/modules/file/lib/Drupal/file/Tests/SaveTest.php

core/modules/file/lib/Drupal/file/Tests/SaveUploadTest.php

core/modules/file/lib/Drupal/file/Tests/SpaceUsedTest.php

core/modules/file/lib/Drupal/file/Tests/UsageTest.php

core/modules/file/lib/Drupal/file/Tests/ValidateTest.php

core/modules/file/lib/Drupal/file/Tests/ValidatorTest.php

...s/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php

core/modules/simpletest/lib/Drupal/simpletest/TestBase.php

...odules/system/lib/Drupal/system/Tests/File/ConfigTest.php

...ules/system/lib/Drupal/system/Tests/File/FileTestBase.php

...ules/system/lib/Drupal/system/Tests/File/MimeTypeTest.php