Introduce pytest check_next_key_event, get_keyids

Introduce pytest check_next_key_event, get_keyids

For the kasp tests we need a new utility that can retrieve a list of
Keys from a given directory, belonging to a specific zone. This is
'keydir_to_keylist' and is the replacement of 'kasp.sh:get_keyids()'.

'next_key_event_eqauls' is a method to check when the next key event is
scheduled, needed for the rollover tests, and is the equivalent of shell
script 'check_next_key_event'.