[9.x] Avoid mutating the $expectedLitener between loops on Event::assertListening

[phcostabh]

After upgrading from Laravel 8.x to 9.x, some of the assertions for the assertListening failed randomly.

I observed that when iteration over the listeners for a given event, if a Class[@]method callback (PostAutoEventSubscriber::class . '@handle') string is evaluated before any class-based listener (PostAutoEventSubscriber::class), it causes the assertion to fail wrongfully.

This happens because after the callback string is evaluated, the $expectedListener is normalized for the comparison. It causes the subsequent iteration to compare the $actualListener with the changed $expectedListener from the previous one.

You can attest that by changing the order in which the listeners are bound in the tests/Integration/Events/EventFakeTest.php from:

Event::listen('event', PostEventSubscriber::class);
Event::listen('event', 'Illuminate\\Tests\\Integration\\Events\\PostAutoEventSubscriber@handle');

To:

Event::listen('event', 'Illuminate\\Tests\\Integration\\Events\\PostAutoEventSubscriber@handle');
Event::listen('event', PostEventSubscriber::class);

To solve the issue, we're just "caching" the normalized form of the expected listeners to avoid mutability between the iteration loops.