Add unit tests for GapBuffer and fix an insert() bug.

src/core/GapBuffer.cc

@@ -70,5 +70,6 @@ void GapBuffer::move_gap(size_t position)
         {
             memmove(&m_buffer[m_gap_position], &m_buffer[m_gap_position + gap_size()], position - m_gap_position);
         }
+        m_gap_position = position;
     }
 }