selftests: drv-net: replace the rpath helper with Path objects

The single letter + "path" helpers do not have many fans (see Link).
Use a Path object with a better name. test_dir is the replacement
for rpath(), net_lib_dir is a new path of the $ksft/net/lib directory.

The Path() class overloads the "/" operator and can be cast to string
automatically, so to get a path to a file tests can do:

    path = env.test_dir / "binary"

Link: https://lore.kernel.org/CA+FuTSemTNVZ5MxXkq8T9P=DYm=nSXcJnL7CJBPZNAT_9UFisQ@mail.gmail.com


Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250327222315.1098596-2-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>