test(fslib): restore timers after each test

[merceyz]

What's the problem this PR addresses?

When calling jest.useFakeTimers we don't always restore the real timers again.

Fixes the timeouts in #4419.

How did you fix it?

Add an afterEach block that calls jest.useRealTimers in the test files that calls jest.useFakeTimers.

Checklist

• I have read the Contributing Guide.
• I have set the packages that need to be released for my changes to be effective.
• I will check that all automated PR checks pass before the PR gets reviewed.

[arcanis]

Wouldn't it be safer to keep using useFakeTime, and to put a jest.useRealTimers inside the finally? Otherwise forgetting to reset the timer sounds an easy mistake 🤔

[merceyz]

Considering some of the tests didn't use useFakeTime I don't think that would help much here, we would have the same problem with "forgetting" to use useFakeTime.

[arcanis]

But it'd be possible to prevent the use of the unsafe Jest function via an ESLint rule (no-restricted-syntax), whereas we can't enforce the reset being called.

Merging anyway since adding such a rule may be a bit complex to do in a generic way, but just something to keep in mind.