Allow a single subscriber to subscribe to multiple event types

[mollierobbert]

PHPUnit's event system could potentially allow a single subscriber to subscribe to multiple events using union types. For example:

class MySubscriber implements PHPUnit\Event\Test\PreparedSubscriber, PHPUnit\Event\Test\FinishedSubscriber
{
    public function notify(PHPUnit\Event\Test\Prepared | PHPUnit\Event\Test\Finished $event): void
    {
        // Do something interesting with either event
    }
}

However, currently in PHPUnit\Event\Dispatcher\DirectDispatcher::registerSubscriber(), it is assumed each subscriber only implements a single subscriber interface.

If you try to add MySubscriber anyway, it will be registered to only one of the two event types (whichever comes first in the PHPUnit\Event\Facade::registerDefaultTypes() list), and thus never receive events of the second type.

Allowing a single subscriber to define multiple subscriber interfaces could help reduce boilerplate in extensions.