Move event handler inheritance test to PHPUnit using mock objects

this is dodgy and we shouldn't rely on it long term.

relates to #6524

tests/phpunit/event/EventTest.php

@@ -0,0 +1,65 @@
+<?php
+
+/*
+ *
+ *  ____            _        _   __  __ _                  __  __ ____
+ * |  _ \ ___   ___| | _____| |_|  \/  (_)_ __   ___      |  \/  |  _ \
+ * | |_) / _ \ / __| |/ / _ \ __| |\/| | | '_ \ / _ \_____| |\/| | |_) |
+ * |  __/ (_) | (__|   <  __/ |_| |  | | | | | |  __/_____| |  | |  __/
+ * |_|   \___/ \___|_|\_\___|\__|_|  |_|_|_| |_|\___|     |_|  |_|_|
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * @author PocketMine Team
+ * @link http://www.pocketmine.net/
+ *
+ *
+ */
+
+declare(strict_types=1);
+
+namespace pocketmine\event;
+
+use PHPUnit\Framework\TestCase;
+use pocketmine\plugin\Plugin;
+use pocketmine\plugin\PluginManager;
+use pocketmine\Server;
+
+final class EventTest extends TestCase{
+
+	public function testHandlerInheritance() : void{
+		//TODO: this is a really bad hack and could break any time if PluginManager decides to access its Server field
+		//we really need to make it possible to register events without a Plugin or Server context
+		$mockServer = $this->createMock(Server::class);
+		$mockPlugin = self::createStub(Plugin::class);
+		$mockPlugin->method('isEnabled')->willReturn(true);
+
+		$pluginManager = new PluginManager($mockServer, null);
+
+		$expectedOrder = [
+			TestGrandchildEvent::class,
+			TestChildEvent::class,
+			TestParentEvent::class
+		];
+		$actualOrder = [];
+
+		foreach($expectedOrder as $class){
+			$pluginManager->registerEvent(
+				$class,
+				function(TestParentEvent $event) use (&$actualOrder, $class) : void{
+					$actualOrder[] = $class;
+				},
+				EventPriority::NORMAL,
+				$mockPlugin
+			);
+		}
+
+		$event = new TestGrandchildEvent();
+		$event->call();
+
+		self::assertSame($expectedOrder, $actualOrder, "Expected event handlers to be called from most specific to least specific");
+	}
+}