Fix attach test on windows.

[Random-Liu]

See https://prow.k8s.io/view/gcs/kubernetes-jenkins/pr-logs/pull/containerd_cri/1264/pull-cri-containerd-windows-cri/1171919947392094208

On windows, if we attach to the container too early, we may get extra output:

C:/Users/prow/AppData/Local/Temp/cri-install-crictl.6mPD/src/github.com/kubernetes-sigs/cri-tools/pkg/validate/streaming.go:109
Timed out after 60.011s.
The stdout of attach should be hello
Expected
    <string>: Microsoft Windows [Version 10.0.17763.737]
    (c) 2018 Microsoft Corporation. All rights reserved.
    
    C:\>hello
    
    C:\>
to equal
    <string>: hello
    
    C:\>
C:/Users/prow/AppData/Local/Temp/cri-install-crictl.6mPD/src/github.com/kubernetes-sigs/cri-tools/pkg/validate/streaming.go:298

We should wait for the output to be stable before feeding in the input command.
Signed-off-by: Lantao Liu lantaol@google.com

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[feiskyer]

/lgtm