[9.18] fix: test: Fix "checking startup notify rate limit" failure

[9.18] fix: test:  Fix "checking startup notify rate limit" failure

Fix the loop terminating condition to get consistent sample sizes and increase the minimum number of samples from 20 to 40.

Closes #5091

Backport of MR !9894

Merge branch 'backport-5091-investigate-checking-startup-notify-rate-limit-failure-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9910