[9.20] chg: dev: Use underscore for system test names

Change the convention for system test directory names to always use an
underscore rather than a hyphen. Names using underscore are valid python
package names and can be used with standard `import` facilities in
python, which allows easier code reuse.

Backport of MR !11710

Merge branch 'backport-nicki/system-test-dir-underscore-names-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11711