[9.20] chg: nil: License test data files in REUSE.toml

In order to avoid the need to repeat the license header in named.conf
files and zone files, add the paths to REUSE.toml to assign the license.

Backport of MR !11766

Merge branch 'backport-nicki/reuse-cover-configs-and-zones-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11773