Rename wxBoxSizer::m_minSize to avoid clash with the base class

wxSizer already has m_minSize field, use m_calculatedMinSize for the field of
the derived wxBoxSizer class to avoid confusion, just as wxFlexGridSizer
already did.

Also add a new unit test checking that GetMinSize() still works after this
change.

include/wx/sizer.h

@@ -1016,7 +1016,7 @@ protected:
 
     // the minimal size needed for this sizer as calculated by the last call to
     // our CalcMin()
-    wxSize m_minSize;
+    wxSize m_calculatedMinSize;
 
 private:
     wxDECLARE_CLASS(wxBoxSizer);