remove ProgressDialog from CI testing

examples/test_examples.py

@@ -70,10 +70,6 @@ conditionalExamples = {
         False,
         reason="Test is being problematic on CI machines"
     ),
-    "ProgressDialog.py": exceptionCondition(
-        not(platform.system() == "Linux"),
-        reason="QXcbConnection: XCB error"
-    ),
     'GLVolumeItem.py': exceptionCondition(
         not(platform.system() == "Darwin" and
             tuple(map(int, platform.mac_ver()[0].split("."))) >= (10, 16) and 

examples/utils.py

@@ -108,7 +108,6 @@ others = dict([
     ('DateAxisItem_QtDesigner', 'DateAxisItem_QtDesigner.py'),
     ('GraphicsScene', 'GraphicsScene.py'),
     ('MouseSelection', 'MouseSelection.py'),
-    ('ProgressDialog', 'ProgressDialog.py'),
 ])
 
 
@@ -118,3 +117,8 @@ trivial = dict([
     ('LogPlotTest', 'LogPlotTest.py'),  # Plotting.py
     ('ViewLimits', 'ViewLimits.py'),    # ViewBoxFeatures.py
 ])
+
+# examples that are not suitable for CI testing
+skiptest = dict([
+    ('ProgressDialog', 'ProgressDialog.py'),    # modal dialog
+])