selftests: drv-net: import things in lib one by one

pylint doesn't understand our path hacks, and it generates a lot
of warnings for driver tests. Import what we use one by one, this
is hopefully not too tedious and it makes pylint happy.

Link: https://patch.msgid.link/20250621171944.2619249-9-kuba@kernel.org


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