Fix bug with wxRadioButton state changing unexpectedly in wxMSW

In wxMSW, a focused wxRadioButton is always checked, which meant that
checking a wxRadioButton while focus was not in the window containing it
and later giving the focus to that window could uncheck it by giving
focus to another wxRadioButton that had had it previously.

Fix this by adding WXSetPendingFocus() to wxMSW wxWindow and calling it
from wxRadioButton::SetValue() to ensure that when the focus is
regained, it goes to the newly checked radio button and not some other
one.

This replaces the previously used, for the same purpose, wxMSW-specific
wxTopLevelWindow::SetLastFocus(), so while this solution is not exactly
pretty, it's not worse than we had before, while being more generic.

Also add a unit test checking that things work correctly in the scenario
described above.

Closes https://github.com/wxWidgets/wxWidgets/pull/1257

Closes #18341.

include/wx/containr.h

@@ -266,6 +266,12 @@ public:
     {
         return m_container.HasTransparentBackground();
     }
+
+    WXDLLIMPEXP_INLINE_CORE
+    virtual void WXDoUpdatePendingFocus(wxWindow* win) wxOVERRIDE
+    {
+        return m_container.SetLastFocus(win);
+    }
 #endif // __WXMSW__
 
 protected: