[9.18] fix: nil: Move root zone mirror system test to a separate directory

This test doesn't require artifact checking but when bundled in the same
directory with the shell based tests, the `system:clang:tsan` job was
failing non-deterministically.

An example of the job failing and succeeding on the same commit:
- https://gitlab.isc.org/isc-projects/bind9/-/jobs/5809299
- https://gitlab.isc.org/isc-projects/bind9/-/jobs/5809447

Backport of MR !10659

Merge branch 'backport-stepan/move-root-zone-mirror-test-to-a-separate-directory-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10663