refactor roundtrip tests
refactor roundtrip tests

shoyer/xray,pydata/xarray,shoyer/xarray,pydata/xarray,jhamman/xarray,xray/xray,jhamman/xray,rabernat/xray,jhamman/xarray,jcmgray/xarray,markelg/xray,markelg/xray,markelg/xray,rabernat/xray,chunweiyuan/xarray,jhamman/xarray,pydata/xarray,shoyer/xarray