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 <[email protected]>

Author: seisman

pygmt/tests/test_grdimage.py

@@ -74,9 +74,13 @@ def test_grdimage_file():
     return fig
 
 
-@pytest.mark.skip(
-    reason="Upstream bug in GMT 6.1.1",
-    condition=gmt_version <= Version("6.1.1") and sys.platform == "darwin",
+@pytest.mark.skipif(
+    gmt_version <= Version("6.1.1") and sys.platform == "darwin",
+    reason="Upstream bug in GMT 6.1.1 that causes segfault on macOS",
+)
+@pytest.mark.xfail(
+    condition=gmt_version <= Version("6.1.1") and sys.platform != "darwin",
+    reason="Upstream bug in GMT 6.1.1 that causes this test to fail on Linux/Windows",
 )
 @check_figures_equal()
 @pytest.mark.parametrize(