Add test for readonly classes implementing an interface property

[iluuu1994]

cf: https://www.reddit.com/r/PHP/comments/13camsm/comment/jjljmqd/

[iluuu1994]

We should add a test to confirm this, but I believe what would happen is a readonly class may implement an interface with properties, as long as it satisfies those properties with basic properties that have no hooks. So, this should be fine:

interface I {
    public string $name { get; }
}

readonly class R implements I {
    public function __construct(public string $name) {}
}

That's not what currently happens. Every property in a readonly class becomes implicitly readonly. However, non-readonly properties may not be re-declared as readonly. If it's simple enough, it makes sense to relax this restriction.

Fatal error: Cannot redeclare non-readonly property I::$name as readonly R::$name

[Crell]

Hrm, interesting. I would also favor relaxing that restriction here if possible. It seems like a reasonable use case.