chore: add test coverage for subdirs

Author: bdraco

src/aiousbwatcher/impl.py

@@ -77,6 +77,7 @@ async def _watcher(self) -> None:
             | Mask.MOVED_TO
             | Mask.CREATE
             | Mask.DELETE_SELF
+            | Mask.DELETE
             | Mask.IGNORED
         )