Do not skip xarray shading tests on all platforms (#669)
Do not skip xarray shading tests on all platforms (#669)

xarray grid shading is known to fail with GMT 6.1.1. 
That's why we want to mark the test xfail. 
The xfail marker works well for Linux and Windows, 
but not for macOS, due to segmentation faults.

So ideally what we should do is:

- GMT 6.1.1 + Linux/Windows: xfail
- GMT 6.1.1 + macOS: skip
- GMT master + Linux/Windows/macOS: no marker

Co-authored-by: Wei Ji <9490087a8b98c7de12813b2e194ef86ac3043a88@users.noreply.github.com>
GenericMappingTools/gmt-python,GenericMappingTools/gmt-python