add unit test for coalescing erase change units within a change operation

src/core/Buffer.h

@@ -137,7 +137,9 @@ public:
     void undo();
     void redo();
 
+#ifndef ENABLE_TESTING
 protected:
+#endif
     bool m_eol_at_eof;
     LineEndings::Type m_line_endings;
     std::shared_ptr<GapBuffer> m_gap_buffer;