2.0 Feature: Make dotnet test explorer UI work similar to Search so devs can work with large numbers of tests

[janaka]

If tests were exposed in a UI similar to search rather than Explorer more powerful UI can be built similar to Visual Studio.

• a user can free text search to see a filtered list of tests + only failing tests. -- run these tests with a keyboard shortcut.

I've not investigated if this is possible. I have a suspicion that VSCode doesn't allow top level menu items to be added as 3rd party extensions. Maybe making this project part of the main project is the only way.

Here's the relevant issue on the VS Code repo, I've added a comment https://github.com/Microsoft/vscode/issues/1833

[janaka]

As we've added a top level option since VS Code has opened that up we can think about improving the experience.