testing: exploratory UI for followup actions

This adds an API that extensions can use to contribute 'followup'
actions around test messages. Here just a dummy hello world using
copilot, but extensions could have any action here, such as actions to
update snapshots if a test failed:

![](https://memes.peet.io/img/24-05-c1f3e073-a2da-4f16-a033-c8f7e5cd4864.png)

Implemented using a simple provider API.

src/vs/workbench/api/common/extHost.api.impl.ts

@@ -453,6 +453,10 @@ export function createApiFactoryAndRegisterActors(accessor: ServicesAccessor): I
 				checkProposedApiEnabled(extension, 'testObserver');
 				return extHostTesting.runTests(provider);
 			},
+			registerTestFollowupProvider(provider) {
+				checkProposedApiEnabled(extension, 'testObserver');
+				return extHostTesting.registerTestFollowupProvider(provider);
+			},
 			get onDidChangeTestResults() {
 				checkProposedApiEnabled(extension, 'testObserver');
 				return _asExtensionEvent(extHostTesting.onResultsChanged);