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 lantaol@google.com

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Random-Liu

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [Random-Liu]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[feiskyer]

/lgtm