Implement allowed extensions filtering in tests

[testlabauto]

Because we are seeing problems with Pyrefly starting up hundreds of times during tests, we think it would be great to implement a settings structure like:

    "extensions.allowed": {
        "meta.pyrefly": false,
        "*": true
    }

This way we can turn off Pyrefly for most tests, but then create a separate suite just for Pyrefly where we wait around a little for the resource spike to pass, then begin testing. This new suite would not be run with parallel execution to avoid the issues we currently face.