fix: test: Support serving signed child zone from its parent's nameserver

When a signed zone is served by the same nameserver instance as
its parent, the child's dnssec-signzone has already written
dsset-<child>. into that directory. Don't attempt to copy the dsset if
the destination and source files are the same.

Assisted-by: Claude:claude-opus-4-8
Merge branch 'nicki/serve-signed-child-same-ns' into 'main'

See merge request isc-projects/bind9!12354