Make system tests compatible with pytest 8.0.0+

Make system tests compatible with pytest 8.0.0+

The pytest collection mechanism has been overhauled in pytest 8.0.0,
resulting in a different node tree when collecting the tests. Ensure the
paths / names we're using that are derived from the node tree are
consistent across different pytest versions.

Particularly, this has affected the convenience symlink name (which is
supposed to be in the form of e.g. dns64_sh_dns64 for the dns64 module
and tests_sh_dns64.py module) and the test name that's logged at the
start of the test, which is supposed to include the system test
directory relative to the root system test directory as well as the
module name (e.g. dns64/tests_sh_dns64.py).

Related https://github.com/pytest-dev/pytest/issues/7777