[9.x] More flexibility when time traveling in tests

[Gimcrack]

Originally targeted 8.x #35326 , was informed it was a breaking change, so switched to 9.x

The purpose of this PR is to bring parity to the InteractsWithTime::travelTo parameter type hint so it matches the Carbon::setTestNow type hint.

IMO this will allow for more flexibility when time traveling in tests. Here are some examples that would be supported if this PR were merged.

Currently, these all result in a type error.

$this->travelTo('Sunday'); 
$this->travelTo('tomorrow');
$this->travelTo('Monday +1 week');
$this->travelTo('+1 week 2 days 4 hours 2 seconds');
$this->travelTo('2020-11-23');

Meanwhile, these all work fine.

Carbon::setTestNow('Sunday'); 
Carbon::setTestNow('tomorrow');
Carbon::setTestNow('Monday +1 week');
Carbon::setTestNow('+1 week 2 days 4 hours 2 seconds');
Carbon::setTestNow('2020-11-23');

Unless I'm missing something, the DateTimeInterface requirement seems like an arbitrary restriction when Carbon's implementation of setTestNow has no such restriction.

// InteractsWithTime.php
public function travelTo(DateTimeInterface $date, $callback = null)
{
    Carbon::setTestNow($date);
    ...
{

// CarbonInterface.php
@param Closure|static|string|false|null $testNow real or mock Carbon instance
public static function setTestNow($testNow = null);

[driesvints]

Please use FQN here for the classes.

[driesvints]

Also please use bool instead of false.