[test] Use fake timers in visual regression tests

[eps1lon]

With #22692 we now have visual regression tests relying on Date which means they produce different output depending on the time they're run.

Using sinon#useFakeTimers to run the tests with the same (mocked) time. This also mocks all other timer related things so the tests should be even "more stable" though we haven't had any timer related flakyness. But it could still mean that we run less timer related stuff (e.g. passive effects?).

We'll see if this works. Otherwise I'll just stub the Date constructor.

[mui-pr-bot]

No bundle size changes

Generated by 🚫 dangerJS against baa5613

[eps1lon]

Looks like it also improved other tests using Date (e.g. docs-getting-started-templates-sign-in/SignIn.png).

Though we now have some inputs in the error state.

Edit:

• docs-components-date-time-picker/DateTimeValidation.png looks ok and having it in the invalid state actually increases coverage.

• docs-components-date-time-picker/CustomDateTimePicker.png should not be invalid as far as I can tell.

[eps1lon]

Let's see if this'll work.