Auto testing can be done only for Flows in the Draft, Ready, or Production stages, and not for Flows in the Archived section.
Case 1: You archived a Flow, but there is still a version in Draft, Ready, or Production.
You can still auto test the Flow.
Case 2: Your archived Flow is the only version on the Dashboard.
You cannot auto test the archived Flow. Any existing test cases for the archived Flow are removed.
Info:
When an archived Flow is restored, its test cases before archival are also restored. You can also create new test cases for the restored Flow if required.