[10.x] Provide testing hooks

[timacdonald]

This PR allows developers to register callbacks to execute when sleeping in tests.

Given the following implementation:

$timeout = now()->addMinute();

do {
    if (Work:attempt()) {
        return;
    }

    Sleep::for(100)->milliseconds();
} while (now()->isAfter($timeout));

When we "fake" sleeping in our tests, the loop will likely do millions of loops, as it doesn't actually pause the code. To test this code we need to be able to progress the now() value.

Introducing Sleep::whenFakingSleep($callback);

$this->freezeTime();
Sleep::fake();
Sleep::whenFakingSleep(fn (Interval $duration) => $this->travel(
    $duration->totalMilliseconds
)->milliseconds());

// run test code.

Sleep::assertSlept();

This solution is inspired by @michaeldyrynda's real-world use case.