e2e: make maxstream test stable
e2e: make maxstream test stable

submitConcurrentWatch use sleep 3s to wait for all the watch connections
ready. When the number of connections increases, like 1000, the 3s is not
enough and the test case becomes flaky.

In this commit, spawn curl process and check the ouput line with
`created":true}}` to make sure that the connection has been initialized
and ready to receive the events. It is reliable to test the following
range request.

Signed-off-by: Wei Fu <df4176d36abdc6fd0d966de5d5471522668de494@gmail.com>

ajityagaty/etcd,ajityagaty/etcd,etcd-io/etcd,etcd-io/etcd,ajityagaty/etcd,etcd-io/etcd