Add common alias cores (x) for grdlandmask (#2944)

Add alias cores (-x) for grdlandmask, and set limit of cores=2
for test_grdlandmask_no_outgrid to make benchmark execution deviate less.

Author: weiji14

pygmt/src/grdlandmask.py

@@ -26,6 +26,7 @@
     R="region",
     V="verbose",
     r="registration",
+    x="cores",
 )
 @kwargs_to_strings(I="sequence", R="sequence", N="sequence", E="sequence")
 def grdlandmask(**kwargs):
@@ -82,6 +83,7 @@ def grdlandmask(**kwargs):
         considered outside [Default is inside].
     {verbose}
     {registration}
+    {cores}
 
     Returns
     -------

pygmt/tests/test_grdlandmask.py

@@ -49,7 +49,7 @@ def test_grdlandmask_no_outgrid(expected_grid):
     """
     Test grdlandmask with no set outgrid.
     """
-    result = grdlandmask(spacing=1, region=[125, 130, 30, 35])
+    result = grdlandmask(spacing=1, region=[125, 130, 30, 35], cores=2)
     # check information of the output grid
     assert isinstance(result, xr.DataArray)
     assert result.gmt.gtype == 1  # Geographic grid