fix data race in integration test: suite tear-down should wait for drain of driver events before proceeding
fix data race in integration test: suite tear-down should wait for drain of driver events before proceeding

mesosphere/mesos-go,mesos/mesos-go,basho-labs/mesos-go,tnachen/go-mesos,chrisjs/mesos-go,ilackarms/mesos-go,jdef/mesos-go,tnachen/mesos-go,drewkerrigan/mesos-go