selftests: netconsole: remove log noise due to socat exit

This removes some noise that can be distracting while looking at
selftests by redirecting socat stderr to /dev/null.

Before this commit, netcons_basic would output:

Running with target mode: basic (ipv6)
2025/11/29 12:08:03 socat[259] W exiting on signal 15
2025/11/29 12:08:03 socat[271] W exiting on signal 15
basic : ipv6 : Test passed
Running with target mode: basic (ipv4)
2025/11/29 12:08:05 socat[329] W exiting on signal 15
2025/11/29 12:08:05 socat[322] W exiting on signal 15
basic : ipv4 : Test passed
Running with target mode: extended (ipv6)
2025/11/29 12:08:08 socat[386] W exiting on signal 15
2025/11/29 12:08:08 socat[386] W exiting on signal 15
2025/11/29 12:08:08 socat[380] W exiting on signal 15
extended : ipv6 : Test passed
Running with target mode: extended (ipv4)
2025/11/29 12:08:10 socat[440] W exiting on signal 15
2025/11/29 12:08:10 socat[435] W exiting on signal 15
2025/11/29 12:08:10 socat[435] W exiting on signal 15
extended : ipv4 : Test passed

After these changes, output looks like:

Running with target mode: basic (ipv6)
basic : ipv6 : Test passed
Running with target mode: basic (ipv4)
basic : ipv4 : Test passed
Running with target mode: extended (ipv6)
extended : ipv6 : Test passed
Running with target mode: extended (ipv4)
extended : ipv4 : Test passed

Signed-off-by: Andre Carvalho <asantostc@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20251129-netcons-socat-noise-v1-1-605a0cea8fca@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>