selftests: net: terminate bkg() commands on exception

selftests: net: terminate bkg() commands on exception

There is a number of:

  with bkg("socat ..LISTEN..", exit_wait=True)

uses in the tests. If whatever is supposed to send the traffic
fails we will get stuck in the bkg(). Try to kill the process
in case of exception, to avoid the long wait.

A specific example where this happens is the devmem Tx tests.

Reviewed-by: Joe Damato <joe@dama.to>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250811231334.561137-5-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>