Use ::class in test mocks of dav app

apps/dav/tests/unit/CardDAV/ConverterTest.php

apps/dav/tests/unit/CardDAV/Sharing/PluginTest.php

apps/dav/tests/unit/Comments/CommentsNodeTest.php

apps/dav/tests/unit/Connector/PublicAuthTest.php

...sts/unit/Connector/Sabre/CommentsPropertiesPluginTest.php

...v/tests/unit/Connector/Sabre/CopyEtagHeaderPluginTest.php

apps/dav/tests/unit/Connector/Sabre/DirectoryTest.php

...tests/unit/Connector/Sabre/DummyGetResponsePluginTest.php

apps/dav/tests/unit/Connector/Sabre/FakeLockerPluginTest.php

apps/dav/tests/unit/Connector/Sabre/FileTest.php

apps/dav/tests/unit/Connector/Sabre/FilesPluginTest.php

.../dav/tests/unit/Connector/Sabre/FilesReportPluginTest.php

apps/dav/tests/unit/Connector/Sabre/NodeTest.php

apps/dav/tests/unit/Connector/Sabre/ObjectTreeTest.php

apps/dav/tests/unit/Connector/Sabre/QuotaPluginTest.php

apps/dav/tests/unit/Connector/Sabre/SharesPluginTest.php

apps/dav/tests/unit/Connector/Sabre/TagsPluginTest.php

apps/dav/tests/unit/DAV/BrowserErrorPagePluginTest.php

apps/dav/tests/unit/DAV/HookManagerTest.php

apps/dav/tests/unit/DAV/Sharing/PluginTest.php