fix(core_test): Check docker.socket instead of docker.service
fix(core_test): Check docker.socket instead of docker.service

docker is now socket activated so the socket is sufficient,
docker.service will be up by the time the docker test finishes but may
or may not be before that.

coreos/coretest,yichengq/coretest,yichengq/coretest,coreos/coretest