sets/resolver: another NSEC delegation to insecure zone

This reproduces the issue encounted in the wild by kresd, reported here:

The main point of the test is to have a delegation from secure to
insecure subzone, when:
- authoritative server for both zones is the same
- the subzone is at least 2 labels below the parent zone apex
11 jobs for sets-delegation-to-insecure in 1 minute and 19 seconds (queued for 7 seconds)