Override wxApp::OnAssertFailure in L7Test.

We do nothing in this override.

wxASSERT_MSG would fail in LGridJsonCellRenderer::ProcessJsonValue and
in XmlHelper::ValidateXmlValue called by
LGridXmlCellRenderer::ProcessXmlValue. This happens with postgresql
only, when the corresponding database value is empty. The cell value
is set to [Invalid], and next rendering leads to a crash in debug build
only.
Why does wxASSERT_MSG fail? Why with postgresql and not with sqlite?

L7Test/L7Test.h

@@ -33,6 +33,8 @@ public:
     
     bool OnInit();
     int OnExit();
+    void OnAssertFailure(const wxChar* file, int line, const wxChar* func, const wxChar* cond, const wxChar* msg) override;
+
 
 private: