tests/periph_wdt: only test powers of two
tests/periph_wdt: only test powers of two

Some watchdog implementations do not support arbitrary precision
for timeouts.
Using powers of two seems like a good common denominator.

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