Test Stabilize TreeView message API

[alexr00]

refs: #80401

• Linux @gregvanl
• Windows @stevencl

Complexity: 2

Test the TreeView message API:

	/**
	 * Represents a Tree view
	 */
	export interface TreeView<T> extends Disposable {
		/**
		 * An optional human-readable message that will be rendered in the view.
		 */
		message?: string;
      }

The tree view example extension creates a view and a command here: https://github.com/microsoft/vscode-extension-samples/blob/master/tree-view-sample/src/testView.ts#L6
You can add to the command to make it change the message.

• Test that you can change the message.
• Test that you can remove the message.
• Read the inline API documentation and see if it is descriptive enough.