Add a description to a vscode.TestTag

Currently when a `vscode.TestTag` is added to a `vscode.TestItem` it will show the label for the `vscode.TestController` that the item is assigned too. It would be helpful if extensions could provide a string label/description for the `vscode.TestTag` that could be presented when the user is auto-completing the tag in the Test filter box like the default ones: <img width="496" alt="Screen Shot 2021-10-04 at 5 22 14 PM" src="https://user-images.githubusercontent.com/22535/135941652-25fc862a-ac3d-48fc-aed8-9366544f03c7.png">