Use fake versions of the standard timer functions. The fakeTimer object is the return value. Contains a side effect of hijacking the globalThis timer.

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

test("should hijack default globalThis timer functions", () => {
  expect(globalThis["setTimeout"]).toEqual(globalThis["setTimeout"]);

  const setTimeout = globalThis["setTimeout"];
  const fakeTimer = useFakeTimer();

  expect(globalThis["setTimeout"]).not.toEqual(setTimeout);

  fakeTimer.useRealTimer();
});