chg: test: Rename "nsec3-delegation" to "nsec3_delegation"

The "nsec3-delegation" test was added in a release branch, before commit
67aca1f8c6f19de11f238a402604697be4ff64dc introduced the current system
test naming convention.  Rename the test to comply with that convention.

Merge branch 'michal/rename-nsec3-delegation-test' into 'main'

See merge request isc-projects/bind9!11753