tests/run-tests: Factor out list supported external boards.
tests/run-tests: Factor out list supported external boards.

To get consistent error messages, etc.

adafruit/circuitpython,adafruit/circuitpython,Peetz0r/micropython-esp32,pfalcon/micropython,SHA2017-badge/micropython-esp32,adafruit/circuitpython,MrSurly/micropython,henriknelson/micropython,blazewicz/micropython,tralamazza/micropython,mhoffma/micropython,bvernoux/micropython,bvernoux/micropython,AriZuu/micropython,torwag/micropython,mhoffma/micropython,pramasoul/micropython,alex-march/micropython,hosaka/micropython,AriZuu/micropython,dinau/micropython,toolmacher/micropython,turbinenreiter/micropython,swegener/micropython,misterdanb/micropython,swegener/micropython,hiway/micropython,MrSurly/micropython-esp32,MrSurly/micropython-esp32,PappaPeppar/micropython,TDAbboud/micropython,dxxb/micropython,blazewicz/micropython,cwyark/micropython,trezor/micropython,oopy/micropython,dinau/micropython,tuc-osg/micropython,jmarcelino/pycom-micropython,pramasoul/micropython,dmazzella/micropython,praemdonck/micropython,deshipu/micropython,tuc-osg/micropython,hiway/micropython,lowRISC/micropython,infinnovation/micropython,cwyark/micropython,pfalcon/micropython,selste/micropython,MrSurly/micropython-esp32,puuu/micropython,hiway/micropython,HenrikSolver/micropython,MrSurly/micropython,jmarcelino/pycom-micropython,alex-robbins/micropython,ryannathans/micropython,praemdonck/micropython,emfcamp/micropython,redbear/micropython,HenrikSolver/micropython,chrisdearman/micropython,Peetz0r/micropython-esp32,trezor/micropython,henriknelson/micropython,PappaPeppar/micropython,mpalomer/micropython,pozetroninc/micropython,tralamazza/micropython,mpalomer/micropython,torwag/micropython,dinau/micropython,TDAbboud/micropython,ganshun666/micropython,oopy/micropython,swegener/micropython,Timmenem/micropython,drrk/micropython,drrk/micropython,adafruit/micropython,micropython/micropython-esp32,ganshun666/micropython,praemdonck/micropython,pramasoul/micropython,Peetz0r/micropython-esp32,Peetz0r/micropython-esp32,matthewelse/micropython,puuu/micropython,turbinenreiter/micropython,selste/micropython,lowRISC/micropython,emfcamp/micropython,mhoffma/micropython,adafruit/micropython,oopy/micropython,MrSurly/micropython,blazewicz/micropython,blazewicz/micropython,infinnovation/micropython,micropython/micropython-esp32,dinau/micropython,emfcamp/micropython,jmarcelino/pycom-micropython,henriknelson/micropython,tobbad/micropython,tobbad/micropython,bvernoux/micropython,toolmacher/micropython,misterdanb/micropython,lowRISC/micropython,trezor/micropython,redbear/micropython,alex-robbins/micropython,Timmenem/micropython,alex-march/micropython,pramasoul/micropython,pozetroninc/micropython,mhoffma/micropython,oopy/micropython,misterdanb/micropython,tuc-osg/micropython,turbinenreiter/micropython,HenrikSolver/micropython,tobbad/micropython,selste/micropython,mpalomer/micropython,turbinenreiter/micropython,alex-robbins/micropython,HenrikSolver/micropython,toolmacher/micropython,misterdanb/micropython,drrk/micropython,micropython/micropython-esp32,ryannathans/micropython,pozetroninc/micropython,redbear/micropython,swegener/micropython,kerneltask/micropython,dmazzella/micropython,pozetroninc/micropython,chrisdearman/micropython,redbear/micropython,praemdonck/micropython,chrisdearman/micropython,Timmenem/micropython,TDAbboud/micropython,jmarcelino/pycom-micropython,hosaka/micropython,MrSurly/micropython-esp32,torwag/micropython,toolmacher/micropython,ryannathans/micropython,pramasoul/micropython,MrSurly/micropython,matthewelse/micropython,TDAbboud/micropython,tuc-osg/micropython,redbear/micropython,torwag/micropython,alex-march/micropython,lowRISC/micropython,hosaka/micropython,dxxb/micropython,TDAbboud/micropython,toolmacher/micropython,PappaPeppar/micropython,matthewelse/micropython,swegener/micropython,ryannathans/micropython,bvernoux/micropython,Peetz0r/micropython-esp32,selste/micropython,torwag/micropython,matthewelse/micropython,Timmenem/micropython,MrSurly/micropython,SHA2017-badge/micropython-esp32,ganshun666/micropython,dxxb/micropython,puuu/micropython,cwyark/micropython,pfalcon/micropython,mpalomer/micropython,Timmenem/micropython,hosaka/micropython,adafruit/circuitpython,tuc-osg/micropython,hosaka/micropython,deshipu/micropython,PappaPeppar/micropython,AriZuu/micropython,bvernoux/micropython,MrSurly/micropython-esp32,hiway/micropython,infinnovation/micropython,blazewicz/micropython,drrk/micropython,infinnovation/micropython,praemdonck/micropython,adafruit/circuitpython,kerneltask/micropython,tralamazza/micropython,adafruit/micropython,AriZuu/micropython,tobbad/micropython,cwyark/micropython,emfcamp/micropython,SHA2017-badge/micropython-esp32,ryannathans/micropython,HenrikSolver/micropython,adafruit/circuitpython,kerneltask/micropython,selste/micropython,oopy/micropython,deshipu/micropython,drrk/micropython,hiway/micropython,kerneltask/micropython,matthewelse/micropython,AriZuu/micropython,dinau/micropython,deshipu/micropython,emfcamp/micropython,pfalcon/micropython,alex-march/micropython,alex-robbins/micropython,henriknelson/micropython,dxxb/micropython,henriknelson/micropython,dmazzella/micropython,infinnovation/micropython,tralamazza/micropython,SHA2017-badge/micropython-esp32,ganshun666/micropython,micropython/micropython-esp32,misterdanb/micropython,trezor/micropython,chrisdearman/micropython,deshipu/micropython,chrisdearman/micropython,alex-march/micropython,mhoffma/micropython,kerneltask/micropython,adafruit/micropython,adafruit/micropython,pozetroninc/micropython,cwyark/micropython,PappaPeppar/micropython,jmarcelino/pycom-micropython,alex-robbins/micropython,dxxb/micropython,SHA2017-badge/micropython-esp32,puuu/micropython,trezor/micropython,mpalomer/micropython,puuu/micropython,ganshun666/micropython,dmazzella/micropython,pfalcon/micropython,micropython/micropython-esp32,matthewelse/micropython,turbinenreiter/micropython,lowRISC/micropython,tobbad/micropython