Expect some failures on Cylindrical Equidistant (Q) plots

Also set RMS tolerance to 1.5 to handle small different along Greenwich Meridian for General Stereographic (S) plots.

pygmt/tests/test_grdimage.py

@@ -109,10 +109,14 @@ def test_grdimage_central_meridians(grid, proj_type, lon0):
     return fig_ref, fig_test
 
 
-@check_figures_equal()
+# Cylindrical Equidistant (Q) projections plotted with xarray and NetCDF grids
+# are still slightly different with an RMS error of 25, see issue at
+# https://github.com/GenericMappingTools/pygmt/issues/390
+# TO-DO remove tol=1.5 and pytest.mark.xfail once bug is solved in upstream GMT
+@check_figures_equal(tol=1.5)
 @pytest.mark.parametrize("lat0", [0, 30])
 @pytest.mark.parametrize("lon0", [0, 123, 180])
-@pytest.mark.parametrize("proj_type", ["Q", "S"])
+@pytest.mark.parametrize("proj_type", [pytest.param("Q", marks=pytest.mark.xfail), "S"])
 def test_grdimage_central_meridians_and_standard_parallels(grid, proj_type, lon0, lat0):
     """
     Test that plotting a grid with different central meridians (lon0) and