Improve kasp test readability

Improve kasp test readability

This commit has a lot of updates on comments, mainly to make the
system test more readable.

Also remove some redundant signing policy checks (check_keys,
check_dnssecstatus, check_keytimes).

Finally, move key time checks and expected key time settings above
'rndc_checkds' calls (with the new way of testing next key event
times there is no need to do them after 'rndc_checkds', and moving
them above 'rndc_checkds' makes the flow of testing easier to follow.

(cherry picked from commit 8cb394e0472da74b66ecc5f695ecb77b3b4dc750)