under-the-hood of test runner: start execution.js

async function runMocha(mocha, options) {
  const { spec = [] } = options;

  // if options.watch watchRun()

  // singleRun
  // collectFiles and lookupFiles here
  mocha.files = spec;
  await mocha.loadFilesAsync();
  return mocha.run(exitMochaLater);
}