selftests/net: Move netkit lease hw setup into per-test fixtures

selftests/net: Move netkit lease hw setup into per-test fixtures

The HW counterpart of nk_qlease.py was carrying its lease setup in main()
and stashing src_queue / nk_queue / nk_*_ifname on cfg, which had drawbacks
called out during the review at [0].

This is the deferred half of the cleanup that landed in commit e254ffb9502c
("selftests/net: Split netdevsim tests from HW tests in nk_qlease") which
was the SW counterpart of nk_qlease.py.

While at it, convert the open-coded "ip netns exec" prefixes in the test
bodies over to the ns= argument of cmd() / bkg().

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>
Link: https://lore.kernel.org/netdev/20260408162238.16709090@kernel.org/
Link: https://patch.msgid.link/20260614102607.863838-2-daniel@iogearbox.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>