related: https://github.com/Microsoft/vscode/issues/5627
added 👇 to .vscode-test.mjs

mocha: {
    ui: 'bdd',
},

// .vscode-test.mjs
import { defineConfig } from '@vscode/test-cli';

export default defineConfig({
    files: 'out/test/**/*.test.js',
    mocha: {
        ui: 'bdd',
    },
});