update ImageGrab tests for python-pillow#6361

Tests/test_imagegrab.py

@@ -1,4 +1,5 @@
 import os
+import shutil
 import subprocess
 import sys
 
@@ -11,7 +12,9 @@
 
 class TestImageGrab:
     @pytest.mark.skipif(
-        sys.platform not in ("win32", "darwin"), reason="requires Windows or macOS"
+        sys.platform not in ("win32", "darwin")
+        and not shutil.which("gnome-screenshot"),
+        reason="requires Windows or macOS, or Linux with gnome-screenshot",
     )
     def test_grab(self):
         ImageGrab.grab()
@@ -22,9 +25,11 @@ def test_grab(self):
         assert im.size == (40, 60)
 
     @skip_unless_feature("xcb")
-    def test_grab_x11(self):
+    def test_grab_xcb(self, monkeypatch):
         try:
             if sys.platform not in ("win32", "darwin"):
+                # test XCB is used when gnome-screenshot is not available
+                monkeypatch.setattr(shutil, "which", lambda x: False)
                 ImageGrab.grab()
 
             ImageGrab.grab(xdisplay="")
@@ -33,11 +38,6 @@ def test_grab_x11(self):
 
     @pytest.mark.skipif(Image.core.HAVE_XCB, reason="tests missing XCB")
     def test_grab_no_xcb(self):
-        if sys.platform not in ("win32", "darwin"):
-            with pytest.raises(OSError) as e:
-                ImageGrab.grab()
-            assert str(e.value).startswith("Pillow was built without XCB support")
-
         with pytest.raises(OSError) as e:
             ImageGrab.grab(xdisplay="")
         assert str(e.value).startswith("Pillow was built without XCB support")