Move WaitForPaint helper into a header

No real changes, just make this class, used in wxWindow::SetSize() tests
only so far, reusable in the other tests.

tests/waitforpaint.h

@@ -0,0 +1,75 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        tests/waitforpaint.h
+// Purpose:     Helper WaitForPaint class
+// Author:      Vadim Zeitlin
+// Created:     2019-10-17 (extracted from tests/window/setsize.cpp)
+// Copyright:   (c) 2019 Vadim Zeitlin <vadim@wxwidgets.org>
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_TESTS_WAITFORPAINT_H_
+#define _WX_TESTS_WAITFORPAINT_H_
+
+#include "wx/stopwatch.h"
+#include "wx/window.h"
+
+// Class used to check if we received the (first) paint event.
+class WaitForPaint
+{
+public:
+    explicit WaitForPaint(wxWindow* win)
+        : m_win(*win),
+          m_painted(false),
+          m_handler(&m_painted)
+    {
+        m_win.Bind(wxEVT_PAINT, m_handler);
+    }
+
+    // This function waits up to the given number of milliseconds for the paint
+    // event to come and returns true if we did get it or false otherwise.
+    bool YieldUntilPainted(int timeoutInMS = 250) const
+    {
+        wxStopWatch sw;
+        for ( ;; )
+        {
+            wxYield();
+
+            if ( m_painted )
+                return true;
+
+            if ( sw.Time() > timeoutInMS )
+                return false;
+        }
+    }
+
+    ~WaitForPaint()
+    {
+        m_win.Unbind(wxEVT_PAINT, m_handler);
+    }
+
+private:
+    wxWindow& m_win;
+    bool m_painted;
+
+    class PaintHandler
+    {
+    public:
+        // Note that we have to use a pointer here, i.e. we can't just store
+        // the flag inside the class itself because it's going to be cloned
+        // inside wx and querying the flag of the original copy wouldtn' work.
+        explicit PaintHandler(bool* painted)
+            : m_painted(*painted)
+        {
+        }
+
+        void operator()(wxPaintEvent& event)
+        {
+            event.Skip();
+            m_painted = true;
+        }
+
+    private:
+        bool& m_painted;
+    } m_handler;
+};
+
+#endif // _WX_TESTS_WAITFORPAINT_H_