I take it there is no way to force MikroTik router to check for updates and update itself without user input. I want daily checks, auto-update to the latest testing version, auto-upgrade RouterBOARD, and reboot at 12:00 AM. Is there a custom script for that?
Use search function on the forum. Simple https://forum.mikrotik.com/search.php?keywords=script+upgrade lets find:
http://forum.mikrotik.com/t/add-custom-tool-for-routerboard-upgrade/181390/1
http://forum.mikrotik.com/t/verify-update-routeros-with-memory/181045/1
Why?
Very good question 
, several possible answers come to my mind 
, OP may:
- have a setup working too well and reliably (boring) and wants to introduce some variations.
- like to be woken in the middle of the night or early in the morning to cries of "there is no internet!" or "the network doesn't work!".
- want to exercise his problem solving skills against new, strange challenges.
- want to break Rule #10 of the just published Mikrotik Club Rules:
The twelve Rules of Mikrotik Club
because rules are made to be broken.
He can say “it’s not my fault” without lying as he slept and did nothing at night. 
I use this script for Auto-update:
:log info “Update-Check started…”
system package update
download once
check-for-updates once
:delay 5s;
:if ( [get status] = “New version is available”) do={ install }