Fix exec test on windows

[Random-Liu]

For containerd/cri#1257

There are several problems with the original test:

1. IIUC, it should use select instead of switch;
2. Only checking len(localOut.String()) >= len(stdout) can't make sure that the exec is done. For tty=true and stdin=true test, closing the stdin will terminate the test exec;
3. The len(localOut.String()) >= len(stdout) logic doesn't work on windows. On windows, when tty is enabled, a lot of control characters will be generated before the actual output, e.g.:

�[2J�[?25l�[m�[H























�[H�]0;C:\Windows\System32\WindowsPowerShell\v1.0\powershell.exe ��[?25h �[H�]0;Administrator: C:\Windows\System32\WindowsPowerShell\v1.0\powershell.exe ��[?25lhello ��[?25h�[?25l�[Hh
�[?25h�[Hh

Signed-off-by: Lantao Liu [email protected]

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[feiskyer]

/lgtm