Add graph plotting for dudect analysis

[SimonLiu423]

Implement graph generation using "gnuplot" for dudect to visualize t-statistic measurements.

• Add "plot.c" and "plot.h" files to support graphing, "plot.c" stores each measurements execution time in "data_buffer". When plot_graph() is called, stores the data to file and generates a "gnuplot" script. I try to replicate the graph shown in "dudect's" paper "Dude, is my code constant time?".

• Modify "fixture.c" to integrate with new plotting functionality.

• Move definition of ENOUGH_MEASURE and TEST_TRIES from "fixture.c" to "fixture.h" for "plot.c" to access and specify size of "data_buffer"

• Update "Makefile" to include plot.o in build

This enhancement allows visual representation of constant-time analysis results, making it easier to interpret dudect test outcomes.