update fixtures in future #113

I think the whole test suite would benefit from a pass through at some point. The geom_array fixture is used repeatedly as a numpy array and then cast to a GeometryArray, it seems, to make testing for equality easier when xarray didn't handle this extension array stuff natively. So perhaps once the minimum version is bumped to a version of xarray where this geometry array type is preserved on the roundtrip, you can make geom_array natively as a fixture an extension array.

Originally posted by @ilan-gold in #112 (comment)