[9.18] fix: test: Resolve the system_test_dir in pytest

If the system_test_dir contains a symlink, then it might cause issues
further down when using relative_to(), unless it is resolved first. This
has been observed on FreeBSD13 in CI where /home is a symlink to
/usr/home.

Backport of MR !11490

Merge branch 'backport-nicki/pytest-freebsd13-artifacts-path-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11495