selftests: net: py: capitalize defer queue and improve import

selftests: net: py: capitalize defer queue and improve import

Import utils and refer to the global defer queue that way instead
of importing the queue. This will make it possible to assign value
to the global variable. While at it capitalize the name, to comply
with the Python coding style.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20260108225257.2684238-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>