Test suite xfail tracking

[tomwhite]

This is a list of work needed to fix the xfails and TODOs in https://github.com/cubed-dev/cubed-xarray/blob/main/cubed_xarray/tests/test_xarray.py

Cubed

• Rechunk where dict has missing axes cubed#546
• Add raise_if_computes for testing cubed#545
• np.nanmean executes eagerly on cubed arrays but lazily on dask arrays cubed#153
• Implement var and std cubed#29

Xarray

• Use duck array ops for around and round pydata/xarray#9326

[More to be added over time...]