tests/run-tests: Make diff tool user configurable.
tests/run-tests: Make diff tool user configurable.

henriknelson/micropython,tobbad/micropython,pramasoul/micropython,tobbad/micropython,adafruit/circuitpython,pramasoul/micropython,kerneltask/micropython,henriknelson/micropython,adafruit/circuitpython,selste/micropython,MrSurly/micropython,MrSurly/micropython,MrSurly/micropython,bvernoux/micropython,MrSurly/micropython,MrSurly/micropython,bvernoux/micropython,bvernoux/micropython,bvernoux/micropython,tobbad/micropython,adafruit/circuitpython,henriknelson/micropython,henriknelson/micropython,kerneltask/micropython,pramasoul/micropython,selste/micropython,kerneltask/micropython,tobbad/micropython,kerneltask/micropython,adafruit/circuitpython,selste/micropython,selste/micropython,adafruit/circuitpython,pramasoul/micropython,bvernoux/micropython,tobbad/micropython,pramasoul/micropython,henriknelson/micropython,selste/micropython,adafruit/circuitpython,kerneltask/micropython