under-the-hood of test runner: load files async.js

Mocha.prototype.loadFilesAsync = async function () {
  var self = this;
  var suite = this.suite;

  for (let file of this.files) {
    // preload
    suite.emit(Suite.constants.EVENT_FILE_PRE_REQUIRE, global, file, self);
    // load
    file = path.resolve(file);
    const result = await require(file);
    // postload events
  }
};