libct/nsenter: test: improve TestNsenterChildLogging
libct/nsenter: test: improve TestNsenterChildLogging

Instead of reading a single message, do read all the logs from the init,
and use DisallowUnknownFields for stricter checking.

While at it, use reapChildren to reap zombies (and add an extra check).

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

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