[9.20] fix: test: nsec3/tests_nsec3_change.py updates zone file too fast

The zone file is updated too soon causing the reload to fail.
Add a 1 second sleep to ensure the modification time has changed.

Closes #5734

Backport of MR !11525

Merge branch 'backport-5734-fix-tests-nsec3-change-py-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11528