kasp tests: fix wait for reconfig done

kasp tests: fix wait for reconfig done

The wait until zones are signed after rndc reconfig is broken
because the zones are already signed before the reconfig.  Fix
by having a different way to ensure the signing of the zone is
complete.  This does require a call to the "wait_for_done_signing"
function after each "check_keys" call after the ns6 reconfig.

The "wait_for_done_signing" looks for a (newly added) debug log
message that named will output if it is done signing with a certain
key.

(cherry picked from commit a47192ed5b11f0dbe7b9054caac1fa5ba690d613)