Add more unit tests for Listener

Author: ShockedPlot7560

tests/phpunit/event/AsyncEventTest.php

@@ -127,4 +127,9 @@ function(TestGrandchildAsyncEvent $event) use (&$firstCompleted, &$run) : ?Promi
 		$resolver->resolve(null);
 		self::assertSame(2, $run, "Expected feedback from 2 handlers");
 	}
+
+	public function testAsyncHandlerFromListener() : void {
+		self::expectNotToPerformAssertions();
+		$this->pluginManager->registerEvents(new TestAsyncListener(), $this->mockPlugin);
+	}
 }

tests/phpunit/event/TestAsyncListener.php

@@ -0,0 +1,45 @@
+<?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 pocketmine\event\fixtures\TestParentAsyncEvent;
+use pocketmine\promise\Promise;
+use pocketmine\promise\PromiseResolver;
+
+class TestAsyncListener implements Listener{
+	public function handleAsyncEventReturningNull(TestParentAsyncEvent $event) : null {
+		return null;
+	}
+
+	public function handleAsyncEventReturningPromise(TestParentAsyncEvent $event) : Promise {
+		/** @var PromiseResolver<null> $resolver */
+		$resolver = new PromiseResolver();
+		$resolver->resolve(null);
+		return $resolver->getPromise();
+	}
+
+	public function handleAsyncEventReturningNullAndPromise(TestParentAsyncEvent $event) : ?Promise {
+		return null;
+	}
+}