[9.18] new: test: Add nsX fixtures to pytest

The nsX are utility fixtures which can be used instead of the servers
fixture, which requires longer syntax(i.e. servers["nsX"]).

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This MR is basically backporting pytest utility fixtures which were introduced to newer branches in !10717. This is a minimal change which only aims to facilitate easier test backports to ~"v9.18" in the future, without changing any of the existing tests.

Merge branch 'nicki/pytest-nsX-fixtures-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11015