refactor: update to new testing API

Previously in the testing API, you called `registerTestProvider` with
your own instance of a TestController, and VS Code would request
workspace or document tests. This has been changed: now, you call
`createTestController`, which returns an object, and call
`createTestItem` to insert test nodes under the `controller.root`.

Extensions should generally decide themselves when to publish tests. For
example, when a file is opened in an editor, test extensions will want
to make sure tests for that file are available so that inline
decorations can be shown. This is pretty similar to what the editor
API does in diagnostics.

There is still a `resolveChildrenHandler` on the controller (rather than
the TestItem directly), which you should _set_ if the test extension
supports lazy discovery. Additionally, if you support running tests,
you'll also want a `runHandler` (migrating from the old `runTests` method).

Some of the existing test providers have been updated, you can check
them out here:

- https://github.com/microsoft/vscode-extension-samples/tree/main/test-provider-sample
- https://github.com/microsoft/vscode-selfhost-test-provider

In summary, to update to the new API:

- Call `vscode.test.createTestController` instead of `registerTestController`
- Move the contents of your `runTests` method to `controller.runHandler`
- Move your `TestItem.resolveHandler` to `controller.resolveChildrenHandler`,
  which may involve adding some `instanceof` checks.
- If you lazily discovered tests in `createDocumentTestRoot`, you'll want
  to trigger that logic based on `vscode.workspace.onDidOpenTextDocument`.
- If your test runner can deal with showing locations of unsaved changes,
  listen for `vscode.workspace.onDidChangeTextDocument` to trigger those
  changes in the tree.

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

@@ -22,7 +22,6 @@ export type ExtHostTestItemEvent =
 
 export interface IExtHostTestItemApi {
 	children: Map<string, TestItemImpl>;
-	parent?: TestItemImpl;
 	bus: Emitter<ExtHostTestItemEvent>;
 }