selftests: drv-net: Test queue xsk attribute

Test that queues which are used for AF_XDP have the xsk nest attribute.
The attribute is currently empty, but its existence means the AF_XDP is
being used for the queue. Enable CONFIG_XDP_SOCKETS for
selftests/drivers/net tests, as well.

Signed-off-by: Joe Damato <jdamato@fastly.com>
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250214211255.14194-4-jdamato@fastly.com


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