fix: test: 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.

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Example of the failure: https://gitlab.isc.org/isc-private/bind9/-/jobs/7773885

Merge branch 'nicki/widen-dnssec-verify-timeout' into 'main'

See merge request isc-projects/bind9!12403