Whatever argument is MockObject or not.

import {
  expect,
  fn,
  isMockObject,
  test,
} from "https://deno.land/x/unitest@$VERSION/mod.ts";

test("should be mock object", () => {
  const mockObject = fn();
  expect(isMockObject(mockObject)).toBeTruthy();
  expect(isMockObject({})).toBeFalsy();
});