[9.20] chg: test: Improve pytest jinja2 templates

- Enable rendering ns-specific data in jinja2 templates using the `ns` varible.
- Add common zone/config snippets an `_common` templates.
- Allow jinja2 imports from `_common`.
- Improve the `_common/controls.conf.j2` snippet to render ns-specific IP rather than hardocded one.

Backport of MR !11805

Merge branch 'backport-nicki/pytest-template-improvements-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!12116