Mark fig.solar tests with xfail

Author: weiji14

pygmt/tests/test_solar.py

@@ -8,6 +8,10 @@
 from pygmt.exceptions import GMTInvalidInput
 
 
+@pytest.mark.xfail(
+    reason="Flaky test only passes with pytest on single module"
+    "See https://github.com/GenericMappingTools/pygmt/issues/1242"
+)
 @pytest.mark.mpl_image_compare
 def test_solar_terminators():
     """
@@ -39,6 +43,10 @@ def test_solar_terminators():
     return fig
 
 
+@pytest.mark.xfail(
+    reason="Flaky test only passes with pytest on single module"
+    "See https://github.com/GenericMappingTools/pygmt/issues/1242"
+)
 @pytest.mark.mpl_image_compare(filename="test_solar_set_terminator_datetime.png")
 @pytest.mark.parametrize(
     "terminator_datetime",
@@ -104,6 +112,10 @@ def test_invalid_datetime():
         )
 
 
+@pytest.mark.xfail(
+    reason="Flaky test only passes with pytest on single module"
+    "See https://github.com/GenericMappingTools/pygmt/issues/1242"
+)
 @pytest.mark.mpl_image_compare(filename="test_solar_set_terminator_datetime.png")
 def test_solar_default_terminator():
     """