Fix the unit test suite exit code in case of failure.

Return the program exit code from the (overridden) OnRun(), not OnExit(),
doing it from the latter didn't work since many years.

The failures in the GUI tests will be detected by buildbot now.

tests/test.cpp

@@ -336,13 +336,15 @@ public:
 
         event.Skip();
     }
-#else // !wxUSE_GUI
+
     virtual int OnRun()
     {
-        m_exitcode = RunTests();
+        if ( TestAppBase::OnRun() != 0 )
+            m_exitcode = EXIT_FAILURE;
+
         return m_exitcode;
     }
-#endif // wxUSE_GUI/!wxUSE_GUI
+#endif // wxUSE_GUI
 
 private:
     void List(Test *test, const string& parent = "") const;
@@ -715,7 +717,7 @@ int TestApp::OnExit()
     delete GetTopWindow();
 #endif // wxUSE_GUI
 
-    return m_exitcode;
+    return TestAppBase::OnExit();
 }
 
 // List the tests