Remove preview flag for VS Code test runner, update tests and remove custom runner

There's a lot of tidying up that can be done when removing the old runner, including:

Removing all the test view/tree classes
Not firing onDidChangeData for the entire tree on many changes
Simplify phantom group/hidden nodes in to TestCoordinator?
Remove the durations from the description (VS Code already adds them)
Remove commands that overlap with new VS Code commands, such as "Run test at cursor" etc.
- This may require better ranges for tests so that Run Test at Cursor works inside the body and not only on the start line
Ensure "Clear Test Results" can still work

Remove legacy test runner

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