[9.18] chg: test: Rename "nsec3-delegation" to "nsec3_delegation"

The "nsec3-delegation" test was added in a release branch, before commit
2441028e724955eb6b1f9aa37884c32ea69c3e5a introduced the current system
test naming convention.  Rename the test to comply with that convention.

Backport of MR !11753

Merge branch 'backport-michal/rename-nsec3-delegation-test-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11755