Wait for the TLW to become iconized under GTK in the test

Iconize() doesn't take effect immediately, so wait until it does for up
to some reasonable amount of time.

tests/persistence/tlw.cpp

@@ -20,6 +20,10 @@
 
 #ifndef WX_PRECOMP
     #include "wx/frame.h"
+
+    #ifdef __WXGTK__
+        #include "wx/stopwatch.h"
+    #endif // __WXGTK__
 #endif // WX_PRECOMP
 
 #include "wx/persist/toplevel.h"
@@ -88,6 +92,7 @@ TEST_CASE_METHOD(PersistenceTests, "wxPersistTLW", "[persist][tlw]")
     }
 
     // Now try recreating the frame using the restored values.
+    bool checkIconized = true;
     {
         wxFrame* const frame = CreatePersistenceTestFrame();
 
@@ -106,6 +111,21 @@ TEST_CASE_METHOD(PersistenceTests, "wxPersistTLW", "[persist][tlw]")
         frame->Iconize();
         frame->Show();
 
+#ifdef __WXGTK__
+        wxStopWatch sw;
+        while ( !frame->IsIconized() )
+        {
+            wxYield();
+            if ( sw.Time() > 500 )
+            {
+                // 500ms should be enough for the window to end up iconized.
+                WARN("Frame wasn't iconized as expected");
+                checkIconized = false;
+                break;
+            }
+        }
+#endif // __WXGTK__
+
         delete frame;
     }
 
@@ -115,8 +135,27 @@ TEST_CASE_METHOD(PersistenceTests, "wxPersistTLW", "[persist][tlw]")
 
         CHECK(wxPersistenceManager::Get().RegisterAndRestore(frame));
 
+        // As above, we need to show the frame for it to be actually iconized.
+        frame->Show();
+
         CHECK(!frame->IsMaximized());
+        if ( checkIconized )
+        {
+#ifdef __WXGTK__
+            wxStopWatch sw;
+            while ( !frame->IsIconized() )
+            {
+                wxYield();
+                if ( sw.Time() > 500 )
+                {
+                    INFO("Abandoning wait after " << sw.Time() << "ms");
+                    break;
+                }
+            }
+#endif // __WXGTK__
+
             CHECK(frame->IsIconized());
+        }
 
         frame->Restore();