add "test" namespace to node.config.json #57559

What is the problem this feature will solve?

I was suggested a few times that a test namespace would be beneficial for the test runner.
It could fix issues like #51384

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

A test namespace with configurations for the test runner, for coverage, filtering etc...
Still need to create a draft of what it could look like

What alternatives have you considered?

none

cc @nodejs/test_runner @cjihrig