selftest: netcons: create a torture test

selftest: netcons: create a torture test

Create a netconsole test that puts a lot of pressure on the netconsole
list manipulation. Do it by creating dynamic targets and deleting
targets while messages are being sent. Also put interface down while the
messages are being sent, as creating parallel targets.

The code launches three background jobs on distinct schedules:

 * Toggle netcons target every 30 iterations
 * create and delete random_target every 50 iterations
 * toggle iface every 70 iterations

This creates multiple concurrency sources that interact with netconsole
states. This is good practice to simulate stress, and exercise netpoll
and netconsole locks.

This test already found an issue as reported in [1]

Link: https://lore.kernel.org/all/20250901-netpoll_memleak-v1-1-34a181977dfc@debian.org/
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Andre Carvalho <asantostc@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20251107-netconsole_torture-v10-3-749227b55f63@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>