Fix PHPUnit 5.4 warnings in DAV app

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

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

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

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

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

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

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

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

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

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

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

...ests/unit/Connector/Sabre/BlockLegacyClientPluginTest.php

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

...ests/unit/Connector/Sabre/CustomPropertiesBackendTest.php

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

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

...av/tests/unit/Connector/Sabre/Exception/ForbiddenTest.php

.../tests/unit/Connector/Sabre/Exception/InvalidPathTest.php

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

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