Move dummy backend to Tests namespace

apps/files_sharing/tests/locking.php

apps/files_sharing/tests/testcase.php

apps/files_versions/tests/versions.php

lib/private/user.php

tests/lib/cache/file.php

tests/lib/files/etagtest.php

tests/lib/files/filesystem.php

tests/lib/files/node/file.php

tests/lib/files/node/folder.php

tests/lib/files/node/integration.php

tests/lib/files/node/node.php

tests/lib/files/node/root.php

tests/lib/files/utils/scanner.php

tests/lib/files/view.php

tests/lib/group.php

tests/lib/preview.php

tests/lib/preview/provider.php

tests/lib/traits/usertrait.php

tests/lib/user.php

tests/lib/user/avataruserdummy.php