testing: update test item async resolving

See https://github.com/microsoft/vscode/issues/126987#issuecomment-867031454
This commit makes the following changes:

- Keep the `resolveChildrenHandler`, and remove the CancellationToken parameter
- Remove `TestITem.status` and instead have `TestItem.canResolveChildren?: boolean`
- Add `TestItem.busy?: boolean`. Note that the UI would implicitly show
  the item as busy while `resolve` is being called. (This is a new feature)

Upgrading to account for these changes should take around 10 to 20 minutes:

1. Where you previously set `item.status = vscode.TestItemStatus.Pending`,
   instead set `item.canResolveChildren = true`.
2. If you used the cancellation token in resolveChildrenHandler, you no
   longer need to do so. What you do here instead is up to you:

	 - If you set up global workspace watchers, add those to `context.subscriptions`
	 - You _probably_ don't need to set up watchers for "file" test items,
	   since you will receive updates via `vscode.workspace.onDidChangeTextDocument`
		 and/or any FileWatcher you have set up.

Example of an update: 7287c64bf7

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

@@ -1273,7 +1273,6 @@ export function createApiFactoryAndRegisterActors(accessor: ServicesAccessor): I
 			NotebookControllerAffinity: extHostTypes.NotebookControllerAffinity,
 			PortAttributes: extHostTypes.PortAttributes,
 			LinkedEditingRanges: extHostTypes.LinkedEditingRanges,
-			TestItemStatus: extHostTypes.TestItemStatus,
 			TestResultState: extHostTypes.TestResultState,
 			TestRunRequest: extHostTypes.TestRunRequest,
 			TestMessage: extHostTypes.TestMessage,

src/vs/workbench/api/common/extHostTypeConverters.ts

@@ -1669,7 +1669,8 @@ export namespace TestItem {
 			uri: URI.revive(item.uri),
 			range: Range.to(item.range || undefined),
 			dispose: () => undefined,
-			status: types.TestItemStatus.Pending,
+			canExpand: false,
+			busy: false,
 			data: undefined as never,
 			debuggable: item.debuggable,
 			description: item.description || undefined,

src/vs/workbench/api/common/extHostTypes.ts

@@ -3297,11 +3297,6 @@ export enum TestMessageSeverity {
 	Hint = 3
 }
 
-export enum TestItemStatus {
-	Pending = 0,
-	Resolved = 1,
-}
-
 const testItemPropAccessor = <K extends keyof vscode.TestItem<never>>(
 	api: IExtHostTestItemApi,
 	key: K,
@@ -3351,7 +3346,8 @@ export class TestItemImpl<T = any> implements vscode.TestItem<T> {
 	public debuggable!: boolean;
 	public label!: string;
 	public error!: string | vscode.MarkdownString;
-	public status!: vscode.TestItemStatus;
+	public busy!: boolean;
+	public canResolveChildren!: boolean;
 
 	constructor(id: string, label: string, uri: vscode.Uri | undefined, public data: T, parent: vscode.TestItem | undefined) {
 		const api = getPrivateApiFor(this);
@@ -3382,7 +3378,8 @@ export class TestItemImpl<T = any> implements vscode.TestItem<T> {
 			description: testItemPropAccessor(api, 'description', undefined, strictEqualComparator),
 			runnable: testItemPropAccessor(api, 'runnable', true, strictEqualComparator),
 			debuggable: testItemPropAccessor(api, 'debuggable', false, strictEqualComparator),
-			status: testItemPropAccessor(api, 'status', TestItemStatus.Resolved, strictEqualComparator),
+			canResolveChildren: testItemPropAccessor(api, 'canResolveChildren', false, strictEqualComparator),
+			busy: testItemPropAccessor(api, 'busy', false, strictEqualComparator),
 			error: testItemPropAccessor(api, 'error', undefined, strictEqualComparator),
 		});