Add withTestNow() for scoped time mocking

[dereuromark]

Summary

• Adds a new static method withTestNow() that temporarily sets the test time, executes a callback, and then restores the previous test time state
• Uses try/finally to ensure state restoration even when callback throws exceptions
• Includes comprehensive tests for basic usage, nested calls, and exception handling

Use Cases

This is useful for:

• Nested time mocking scenarios
• Tests that need to temporarily override a global mock time set in setUp()
• Clean time mocking that automatically restores state

Example

// Basic usage
$result = Chronos::withTestNow('2023-06-15 12:00:00', function () {
    return Chronos::now()->format('Y-m-d');
});
// $result === '2023-06-15'

// Nested calls work correctly
Chronos::setTestNow('2020-01-01');
Chronos::withTestNow('2023-06-15', function () {
    // Here now is 2023-06-15
    Chronos::withTestNow('2024-12-25', function () {
        // Here now is 2024-12-25
    });
    // Back to 2023-06-15
});
// Back to 2020-01-01

Related

Inspired by Carbon's withTestNow() implementation and the fix in briannesbitt/Carbon#3283 which ensures the previous test time is restored (not unconditionally reset to null).