License test data files in REUSE.toml

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.

All shared snippets in bin/tests/system/_common and
bin/tests/system/**named.conf* files are covered.

Note that the existing rule **/**.db** already covers zone files. Only
remove the extraneous special-cases.

(cherry picked from commit ef57bd3a02ff458e8c542ba9447f15106aae3a60)