python311Packages.dask: provide dataframe extra for tests

Tests would previously error out with:

> ImportError: Dask dataframe requirements are not installed.

pkgs/development/python-modules/dask/default.nix

···
     # from panda[test]
     hypothesis
     pytest-asyncio
-  ] ++ lib.optionals (!arrow-cpp.meta.broken) [ # support is sparse on aarch64
+  ]
+  ++ passthru.optional-dependencies.dataframe
+  ++ lib.optionals (!arrow-cpp.meta.broken) [ # support is sparse on aarch64
     pyarrow
   ];