selftests: drv-net: factor out a DrvEnv base class

We have separate Env classes for local tests and tests with a remote
endpoint. Make it easier to share the code by creating a base class.
Make env loading a method of this class.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20250207184140.1730466-1-kuba@kernel.org


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