testrunner: allow setting custom delay before reset
testrunner: allow setting custom delay before reset

On some boards, such as hifive1b, resetting right after flashing could crash the running firmware

yogo1212/RIOT,yogo1212/RIOT,toonst/RIOT,toonst/RIOT,OlegHahm/RIOT,ant9000/RIOT,ant9000/RIOT,jasonatran/RIOT,kYc0o/RIOT,yogo1212/RIOT,x3ro/RIOT,miri64/RIOT,kaspar030/RIOT,authmillenon/RIOT,OTAkeys/RIOT,kaspar030/RIOT,authmillenon/RIOT,yogo1212/RIOT,mtausig/RIOT,jasonatran/RIOT,basilfx/RIOT,OlegHahm/RIOT,yogo1212/RIOT,x3ro/RIOT,toonst/RIOT,kYc0o/RIOT,basilfx/RIOT,authmillenon/RIOT,toonst/RIOT,ant9000/RIOT,authmillenon/RIOT,OlegHahm/RIOT,miri64/RIOT,smlng/RIOT,OTAkeys/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,mtausig/RIOT,ant9000/RIOT,OlegHahm/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,toonst/RIOT,miri64/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,smlng/RIOT,yogo1212/RIOT,smlng/RIOT,ant9000/RIOT,OlegHahm/RIOT,x3ro/RIOT,miri64/RIOT,OTAkeys/RIOT,x3ro/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,jasonatran/RIOT,authmillenon/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,jasonatran/RIOT,x3ro/RIOT,smlng/RIOT,basilfx/RIOT,basilfx/RIOT,authmillenon/RIOT,basilfx/RIOT,mtausig/RIOT,kYc0o/RIOT,miri64/RIOT,kaspar030/RIOT,mtausig/RIOT,smlng/RIOT,mtausig/RIOT