fix: test: Fix a race condition in dnssec test

When dumpdb command is executed, it might take a while until the file is
written. Rather than checking the file once, use the WatchLog mechanism
to allow the desired line to appear before a timeout happens.

This affected test_validation_recovery and test_cache tests which have
been intermittently failing on EL8 in our CI.

Merge branch 'nicki/fix-dnssec-test-dumpdb-race' into 'main'

See merge request isc-projects/bind9!11487