selftests: drv-net-hw: support using Python from net hw tests

We created a separate directory for HW-only tests, recently.
Glue in the Python test library there, Python is a bit annoying
when it comes to using library code located "lower"
in the directory structure.

Reuse the Env class, but let tests require non-nsim setup.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240429144426.743476-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>