Re-define push_back() in wxSortedArrayString to behave correctly.

Adding items to wxSortedArrayString should always keep them sorted but while
Add() did this, push_back() didn't breaking the class invariant.

Redefine push_back() in _WX_DEFINE_SORTED_TYPEARRAY_2 macro to fix this and
add a unit test checking that wxSortedArrayString::push_back() does work now.

Closes #13134.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67429 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/dynarray.h

@@ -546,6 +546,8 @@ public:                                                               \
                                                                       \
   size_t Add(T lItem)                                                 \
     { return base::Add(lItem, (CMPFUNC)m_fnCompare); }                \
+  void push_back(T lItem)                                             \
+    { Add(lItem); }                                                   \
                                                                       \
   void RemoveAt(size_t uiIndex, size_t nRemove = 1)                   \
     { base::erase(begin() + uiIndex, begin() + uiIndex + nRemove); }  \