tests: fix fail with slow semcheck of DNSSEC with valgrind

tests-extra/tests/basic/nsec3/test.py

tests-extra/tests/catalog/rapid_updates/test.py

tests-extra/tests/ixfr/knot_knot/test.py

tests-extra/tests/modules/dnsproxy/test.py

tests-extra/tests/modules/stats/test.py

tests-extra/tests/zone/backup/test.py

tests-extra/tests/zone/backup_lock/test.py

tests-extra/tests/zone/flush_immediate/test.py