selftests: net: py: handle interrupt during cleanup

selftests: net: py: handle interrupt during cleanup

Following up on the old discussion [1]. Let the BaseExceptions out of
defer()'ed cleanup. And handle it in the main loop. This allows us to
exit the tests if user hit Ctrl-C during defer().

Link: https://lore.kernel.org/20251119063228.3adfd743@kernel.org
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20251128004846.2602687-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>