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

The "nsec3-delegation" test was added in a release branch, before commit
e40db975d9917f07da81ae01bfbe8f9cf3b0cab2 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-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11754