benchtests: New -g option to generate graphs in compare_strings.py
The compare_strings.py option unconditionally generates a graph PNG
image of the input data, which can be unnecessary and slow. Put this
behind an optional flag -g.
* benchtests/scripts/compare_strings.py: New option -g.
(draw_graph): Print a message that a graph is being generated.
(process_results): Generate graph only if -g is passed.
(main): Process option -g.