Test TreeView events

[sandy081]

Refs: #49511 #30535

• anyOS: @jrieken
• anyOS: @rebornix

Complexity: 2

Write an extension that contributes a tree view to the Explorer. Refer to documentation and [samples] (https://github.com/Microsoft/vscode-extension-samples/tree/master/tree-view-sample).

Now you can listen to following Treeview events

/**
 * Event that is fired when an element is expanded
 */
readonly onDidExpandElement: Event<T>;

/**
 * Event that is fired when an element is collapsed
 */
readonly onDidCollapseElement: Event<T>;

/**
 * Currently selected elements.
 */
readonly selection: ReadonlyArray<T>;

Test that above events are getting triggered correctly when an element is expanded or collapsed or selected. Also verify that correct data is passed to the call back.