tests: Add option to not clear MICROPYPATH when running tests
tests: Add option to not clear MICROPYPATH when running tests

This allows using the test runner for other scenarios than just
testing uPy itself.
The principle of comparing either to CPython or else to a .exp
file is really handy but to be able to test custom modules not
built into micropython.exe one needs to be able to specify the
module search path a.k.a MICROPYPATH.

puuu/micropython,micropython/micropython-esp32,cwyark/micropython,lowRISC/micropython,ryannathans/micropython,trezor/micropython,Timmenem/micropython,deshipu/micropython,adafruit/circuitpython,micropython/micropython-esp32,deshipu/micropython,Peetz0r/micropython-esp32,oopy/micropython,PappaPeppar/micropython,SHA2017-badge/micropython-esp32,henriknelson/micropython,Timmenem/micropython,tobbad/micropython,TDAbboud/micropython,ryannathans/micropython,hiway/micropython,adafruit/micropython,pfalcon/micropython,puuu/micropython,trezor/micropython,selste/micropython,deshipu/micropython,blazewicz/micropython,oopy/micropython,adafruit/circuitpython,adafruit/micropython,toolmacher/micropython,selste/micropython,lowRISC/micropython,tobbad/micropython,pfalcon/micropython,MrSurly/micropython-esp32,bvernoux/micropython,adafruit/circuitpython,MrSurly/micropython,TDAbboud/micropython,puuu/micropython,adafruit/circuitpython,dmazzella/micropython,henriknelson/micropython,puuu/micropython,pozetroninc/micropython,SHA2017-badge/micropython-esp32,PappaPeppar/micropython,HenrikSolver/micropython,pfalcon/micropython,MrSurly/micropython,infinnovation/micropython,MrSurly/micropython,infinnovation/micropython,henriknelson/micropython,swegener/micropython,cwyark/micropython,torwag/micropython,AriZuu/micropython,bvernoux/micropython,hiway/micropython,adafruit/micropython,tobbad/micropython,dmazzella/micropython,tralamazza/micropython,blazewicz/micropython,alex-robbins/micropython,chrisdearman/micropython,kerneltask/micropython,ryannathans/micropython,pramasoul/micropython,adafruit/micropython,adafruit/circuitpython,micropython/micropython-esp32,infinnovation/micropython,deshipu/micropython,bvernoux/micropython,blazewicz/micropython,henriknelson/micropython,alex-robbins/micropython,lowRISC/micropython,SHA2017-badge/micropython-esp32,HenrikSolver/micropython,PappaPeppar/micropython,chrisdearman/micropython,tobbad/micropython,henriknelson/micropython,torwag/micropython,ryannathans/micropython,adafruit/micropython,cwyark/micropython,PappaPeppar/micropython,tralamazza/micropython,kerneltask/micropython,lowRISC/micropython,torwag/micropython,adafruit/circuitpython,kerneltask/micropython,cwyark/micropython,MrSurly/micropython-esp32,infinnovation/micropython,MrSurly/micropython,selste/micropython,TDAbboud/micropython,tralamazza/micropython,Peetz0r/micropython-esp32,dmazzella/micropython,MrSurly/micropython-esp32,toolmacher/micropython,tralamazza/micropython,oopy/micropython,Peetz0r/micropython-esp32,toolmacher/micropython,chrisdearman/micropython,HenrikSolver/micropython,hiway/micropython,swegener/micropython,trezor/micropython,SHA2017-badge/micropython-esp32,toolmacher/micropython,ryannathans/micropython,pfalcon/micropython,pozetroninc/micropython,kerneltask/micropython,pozetroninc/micropython,AriZuu/micropython,deshipu/micropython,hiway/micropython,PappaPeppar/micropython,chrisdearman/micropython,swegener/micropython,kerneltask/micropython,TDAbboud/micropython,pramasoul/micropython,selste/micropython,selste/micropython,pramasoul/micropython,lowRISC/micropython,TDAbboud/micropython,bvernoux/micropython,Timmenem/micropython,MrSurly/micropython,MrSurly/micropython-esp32,dmazzella/micropython,blazewicz/micropython,trezor/micropython,Timmenem/micropython,alex-robbins/micropython,AriZuu/micropython,tobbad/micropython,pfalcon/micropython,pozetroninc/micropython,alex-robbins/micropython,HenrikSolver/micropython,Peetz0r/micropython-esp32,Timmenem/micropython,infinnovation/micropython,pramasoul/micropython,micropython/micropython-esp32,MrSurly/micropython-esp32,alex-robbins/micropython,pramasoul/micropython,hiway/micropython,cwyark/micropython,swegener/micropython,toolmacher/micropython,puuu/micropython,Peetz0r/micropython-esp32,trezor/micropython,AriZuu/micropython,oopy/micropython,oopy/micropython,chrisdearman/micropython,swegener/micropython,AriZuu/micropython,torwag/micropython,torwag/micropython,SHA2017-badge/micropython-esp32,blazewicz/micropython,HenrikSolver/micropython,pozetroninc/micropython,micropython/micropython-esp32,bvernoux/micropython