testing: remove data from TestItems

Data was viral and spread generics all over the place. It was also
hard to type correctly and consistently, which ended up in the type
being `any` unless great care was taken.

This removes the `data`. Instead, consumers can use a `WeakMap<TestItem, T>`
to keep data associated with the test item. This is a similar pattern
to what is used to store data about documents and debug sessions, for
example. Here's an example of a migration:

8fdf822985 (diff-2fe3ad6ad19447c57c5db14c5a6ccb5544944494db6b909540d70ea499784b49R9)

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

@@ -18,7 +18,7 @@ export type ExtHostTestItemEvent =
 	| [evt: ExtHostTestItemEventType.NewChild, item: TestItemImpl]
 	| [evt: ExtHostTestItemEventType.Disposed]
 	| [evt: ExtHostTestItemEventType.Invalidated]
-	| [evt: ExtHostTestItemEventType.SetProp, key: keyof vscode.TestItem<never>, value: any];
+	| [evt: ExtHostTestItemEventType.SetProp, key: keyof vscode.TestItem, value: any];
 
 export interface IExtHostTestItemApi {
 	children: Map<string, TestItemImpl>;