I think a suppressAllIssues function should be part of a separate proposal. I additionally think that a suppressAllIssues function would be something available under the ForToolsIntegrationOnly spi, and probably expanded to return the list of all issues captured. We absolutely do not want to give Test Authors an out for ignoring issues (which is why withKnownIssue behaves the way it does), but I agree that there are cases when tools authors will write code that generates issues which need to be ignored.
Polling makes sense to include in Swift Testing because it's the most basic part of the "wait for something to update" feature set, which the confirmation API started to introduce.