libct/int/checkpoint_test: use kill(0) for pid check
libct/int/checkpoint_test: use kill(0) for pid check

os.FindProcess never returns an error on Unix/Linux.
Use kill(0) to actually check if the process exists.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

dqminh/runc,opencontainers/runc,rhatdan/runc,opencontainers/runc,rhatdan/runc,kolyshkin/runc,dqminh/runc,rhatdan/runc,kolyshkin/runc,kolyshkin/runc,rhatdan/runc,dqminh/runc,dqminh/runc,kolyshkin/runc,opencontainers/runc,opencontainers/runc