[9.18] fix: test: Require explicit import of isctest.name in system tests

Since the isctest.name module uses `pytest.importorskip()` to check for
dnspython version, it can't be imported automatically on `import
isctest`, because that is used in conftest.py, causing an error during
test setup.

Note that this behavior only manifested on AlmaLinux 8, so perhaps newer
pytest versions are able to handle this edge case more gracefully.

This doesn't affect 9.20+, as this issue was introduced as an erroneous
conflict resolution during a 9.18 backport.

Merge branch 'nicki/fix-isctest-name-import-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10822