Create Python Test TreeViewProvider & TreeViewItem. #4273

Create Python Test TreeViewProvider, Python Test TreeViewItem, and show them in the Test Explorer view.

This task is specifically to create the model and prove out the tree item flow we will use. Dummy data can be created, and unit tests put in place to ensure the tree model update will work how we expect it to.

No commands, nothing that would alter the tree is included in this task.

The end result will be an API that will have to be implemented rather than a full reconfiguration of how we represent this information internally today (Tests data structure, ITestCollectionStorageService, etc...).

NOTE: Discussion on mapping the Tests data structure should be had to ensure we understand any impact of altering or extending this.

NOTE: We are not going to support multi-root work spaces at this time.

Test for completion:

• Examples of all test elements we support can be observed in the Test activity tree.
  • Package, File, Suite, Function