Extend tests for signed, CNAME-sourced delegations

This MR is a follow-up to !5155 (merged). It does not introduce any code changes, but rather only:

• updates the chain system test to make sure we test the contents of signed, secure delegations prepared using both wildcard expansion and CNAME chaining (the second check added by this MR should fail with commit 7a87bf46 reverted),

• updates code comments to prevent confusion.

Closes #2759 (closed)