appengine/aetest: fix datarace
appengine/aetest: fix datarace

Make sure that startChild has stopped reading stderr from the
dev_appserver process before it returns. This prevents a race from
happening when Close is called.

Change-Id: I14a070465ea43d837eed91007db3ddbace433b27

golang/appengine,golang/appengine,fuchsia-mirror/third_party-github.com-golang-appengine,fuchsia-mirror/third_party-github.com-golang-appengine