bug: runWithRealTimers activates fake Timers

[ph-fritsche]

• @testing-library/dom version: master = v7.29.4

Relevant code or config:

https://github.com/ph-fritsche/dom-testing-library/blob/24ebf282d10782b3d0a22b939a1b2950d4b5bd1b/src/__tests__/helpers.js#L86-L118

What you did:

Tried to fix #884

What happened:

The test leaks setTimeout only if jest.useFakeTimers('modern') has been used in some other test before.

Reproduction:

See relevant code above and parent commits.

Problem description:

Stopping the leak is not sufficient as the side-effect is in the tested unit.

The assumption that jest.getRealSystemTime() could be used to determine if a modern jest fake timer is active is wrong.

dom-testing-library/src/helpers.js

Lines 43 to 45 in 5bc9364

	// jest.getRealSystemTime is only supported for Jest's `modern` fake timers and otherwise throws
	jest.getRealSystemTime()
	return 'modern'

Suggested solution:

Determine if a jest fake timer is active by calling jest.useRealTimers() and observing the change of global timer functions.