selftests: net: support use of NetdevSimDev under "with" in python

Using "with" on an entire driver test env is supported already,
but it's also useful to use "with" on an individual nsim.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/20240412141436.828666-6-kuba@kernel.org


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