Testing: regarding to refreshHandler

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> Hello, I'm coming from [this issue](https://github.com/microsoft/vscode/issues/141182). So my user's issue is that when I'm rediscovering automatically (based on file system watcher events or config change) the tests I can not tell the user that rediscovery is happening. I'm aware that I can flip the `is_busy` on a TestItem but the problem is that if there are new test files under discovery there is no TestItem for them. So what we are looking for is a "global" `is_busy` which would turn the refresh button to stop button and would show that progress bar thingy. cc: @connor4312 , @stefanhaller