[9.x] Adds "freezeTime" helper for tests

[DarkGhostHunter]

What?

The freezeTime() method in tests will freeze the current time.

public function test_something()
{
    $this->freezeTime();
}

This is equivalent to $this->travelTo(Carbon::now()).

This PR also adds freezeSecond() which does the same, but sets the time at the start of the current second. This fixes any problem when comparing dates that have no sub-second precision, like the default database timestamps.

public function test_something()
{
    $this->freezeSecond();

    User::factory()->createOne();

    static::assertEquals(now(), User::find(1)->created_at);
}

Why?

Just better syntax.