Extend the check_dnssec_verify retry window to 60 seconds

Extend the check_dnssec_verify retry window to 60 seconds

A fixed ten-iteration poll could be too short for a freshly signed
zone to become fully valid on a heavily loaded host, occasionally
yielding a spurious "zone not verified".

Bump the budget to a 60-second window and, in the process, replace the
hand-rolled retry loop with the standard isctest.run.retry_with_timeout
helper already used by check_next_key_event.

Assisted-by: Claude:claude-opus-4-8