Add waiter in expectation

What does this MR do and why?

What: Add wait method in dashboard images spec.
Why: The dashboard images spec will fail due to FF cache. We follow this comment's advice.

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

/cc @daveliu