selftests: net: lib: Add kill_process

A number of selftests run processes in the background and need to kill them
afterwards. Instead for everyone to open-code the kill / wait / redirect
mantra, add a helper in net/lib.sh. Convert existing open-code sites.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Link: https://patch.msgid.link/a9db102067d741c118f0bd93b10c75e2a34665ea.1731589511.git.petrm@nvidia.com


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