tests: Add a not_xvfb marker.

pytest.ini

@@ -8,6 +8,7 @@ markers =
     osx: Tests which only can run on OS X.
     not_osx: Tests which can not run on OS X.
     not_frozen: Tests which can't be run if sys.frozen is True.
+    not_xvfb: Tests which can't be run with Xvfb.
     frozen: Tests which can only be run if sys.frozen is True.
     integration: Tests which test a bigger portion of code, run without coverage.
 flakes-ignore =

tests/conftest.py

@@ -54,6 +54,8 @@ def _apply_platform_markers(item):
             "Can't be run when frozen"),
         ('frozen', not getattr(sys, 'frozen', False),
             "Can only run when frozen"),
+        ('not_xvfb', item.config.xvfb_display is not None,
+            "Can't be run with Xvfb."),
     ]
 
     for searched_marker, condition, default_reason in markers: