add presuspend sanity check

[zhongkechen]

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Description

Just before go suspend, we do sanity check to make sure:

• it's not suspended more than once
• there is at least one operation pending completion, otherwise, the backend will throw an exception

This change is to help us detect possible issues with suspend in our test cases, not to prevent them.

Demo/Screenshots

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• I have thoroughly tested this change

Testing

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