Fix statschannel system test

Fix statschannel system test

The manykeys test case relies on keys being removed. Make sure the
zone is fully signed with the keys that will stay, so the other keys
may be removed safely.

This means the expected number of signatures generated and refreshed
will change. The CDS and CDNSKEY RRset also need to be signed now.

Configure the test case with sig-signing-signatures 100, large enough
that the entire zone is processed in a single step.