Disable tests requiring a WM under Xvfb

Both iconization and activation of TLWs is dependent on the presence of
a WM, but there is no WM running under Xvfb, so skip these tests in this
case.

tests/persistence/tlw.cpp

@@ -112,6 +112,14 @@ TEST_CASE_METHOD(PersistenceTests, "wxPersistTLW", "[persist][tlw]")
         frame->Show();
 
 #ifdef __WXGTK__
+        // When using Xvfb, the frame will never get iconized, presumably
+        // because there is no WM, so don't even bother waiting or warning.
+        if ( IsRunningUnderXVFB() )
+        {
+            checkIconized = false;
+        }
+        else
+        {
             wxStopWatch sw;
             while ( !frame->IsIconized() )
             {
@@ -124,6 +132,7 @@ TEST_CASE_METHOD(PersistenceTests, "wxPersistTLW", "[persist][tlw]")
                     break;
                 }
             }
+        }
 #endif // __WXGTK__
 
         delete frame;

tests/toplevel/toplevel.cpp

@@ -72,7 +72,11 @@ static void TopLevelWindowShowTest(wxTopLevelWindow* tlw)
     tlw->Show(true);
     countActivate.WaitEvent();
 
+    // TLWs never become active when running under Xvfb, presumably because
+    // there is no WM there.
+    if ( !IsRunningUnderXVFB() )
         CHECK(tlw->IsActive());
+
     CHECK(tlw->IsShown());
 
     tlw->Hide();