selftests: drv-net: add a warning for bkg + shell + terminate

Joe Damato reports that some shells will fork before running
the command when python does "sh -c $cmd", while bash on my
machine does an exec of $cmd directly.

This will have implications for our ability to terminate
the child process on various configurations of bash and
other shells. Warn about using

	bkg(... shell=True, termininate=True)

most background commands can hopefully exit cleanly (exit_wait).

Link: https://lore.kernel.org/Z7Yld21sv_Ip3gQx@LQ3V64L9R2


Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Acked-by: Joe Damato <jdamato@fastly.com>
Tested-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20250219234956.520599-2-kuba@kernel.org


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