feat(jest-fake-timers): Add feature to enable automatically advancing…

[atscott]

… timers

Summary

Testing with mock clocks can often turn into a real struggle when dealing with situations where some work in the test is truly async and other work is captured by the mock clock.

In addition, when using mock clocks, testers are always forced to write tests with intimate knowledge of when the mock clock needs to be ticked. Oftentimes, the purpose of using a mock clock is to speed up the execution time of the test when there are timeouts involved. It is not often a goal to test the exact timeout values. This can cause tests to be riddled with manual advancements of fake time. It ideal for test code to be written in a way that is independent of whether a mock clock is installed or which mock clock library is used. For example:

document.getElementById('submit');
// https://testing-library.com/docs/dom-testing-library/api-async/#waitfor
await waitFor(() => expect(mockAPI).toHaveBeenCalledTimes(1))

When mock clocks are involved, the above may not be possible if there is some delay involved between the click and the request to the API. Instead, developers would need to manually tick the clock beyond the delay to trigger the API call.

This commit attempts to resolve these issues by adding a feature which allows jest to advance timers automatically with the passage of time, just as clocks do without mocks installed.

Test plan

I wrote some unit tests but let me know if you need more.