selftests: drv-net: add netkit devmem tests

Add nk_devmem.py with four tests for TCP devmem through a netkit device.

These tests are just duplicates of the original devmem tests, with some
adjusted

selftests: drv-net: add netkit devmem tests

Add nk_devmem.py with four tests for TCP devmem through a netkit device.

These tests are just duplicates of the original devmem tests, with some
adjusted parameters such as telling ncdevmem to avoid device setup
(since it only has access to netkit, not a phys device).

Each test uses NetDrvContEnv with primary_rx_redirect=True to set up the
BPF redirect program on the primary netkit interface, then calls a
shared run_*() helper which probes for devmem support and configures
the NIC (HDS, RSS, queue lease) before driving the test. NIC state is
restored per-test via defer() callbacks registered inside the helper.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
Link: https://patch.msgid.link/20260514-tcp-dm-netkit-v5-8-408c59b91e66@meta.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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