What is the problem this feature will solve?

Having a stable test runner in core. People have been asking for it, and I feel the majority of the test runner is ready.

What is the feature you are proposing to solve the problem?

I would like to stabilize the test runner for the 20.0.0 release. This currently would not include code coverage, which is behind a separate flag.

What alternatives have you considered?

Not stabilizing the test runner.