Add action buttons to top of Test Explorer

[d3r3kk]

Hook up Run Tests/Discover Tests/Debug Tests/Run Failed Tests, commands to the Test Explorer title bar.

These commands will act on the entire tree (There are sub-tree tasks following). There are Run Failed commands that exist and run against the entire workspace today.

Test for completion:

• The necessary buttons appear on the test explorer
• The ellipsis button appears on the test explorer
• The ellipsis button expands to a menu when clicked
• The buttons work the same as if the command-palette commands are issued today.

[d3r3kk]

Informational Links

Overview of views and how we contribute to them:
https://code.visualstudio.com/api/references/contribution-points#contributes.views

Tree View Sample:
https://github.com/Microsoft/vscode-extension-samples/tree/master/tree-view-sample

[DonJayamanne]

The ellipsis button appears on the test explorer

This is missing

[karrtikr]

The ellipsis button appears on the test explorer

This is missing

We decided not to do that, because the icons can easily fit in the test explorer title bar.