tests/periph_rtt: be more verbose on error
tests/periph_rtt: be more verbose on error

If `rtt_get_alarm()` doesn't return the correct value, print expected
and return value to ease debugging.

authmillenon/RIOT,OlegHahm/RIOT,ant9000/RIOT,kYc0o/RIOT,authmillenon/RIOT,OlegHahm/RIOT,miri64/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,ant9000/RIOT,authmillenon/RIOT,kYc0o/RIOT,jasonatran/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,jasonatran/RIOT,OTAkeys/RIOT,kaspar030/RIOT,OlegHahm/RIOT,miri64/RIOT,jasonatran/RIOT,jasonatran/RIOT,miri64/RIOT,OTAkeys/RIOT,kaspar030/RIOT,jasonatran/RIOT,ant9000/RIOT,OlegHahm/RIOT,authmillenon/RIOT,kYc0o/RIOT,authmillenon/RIOT,OTAkeys/RIOT,authmillenon/RIOT,miri64/RIOT,kYc0o/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,miri64/RIOT,RIOT-OS/RIOT,ant9000/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT