Random: fixed nextSignedInt() not actually returning signed ints

closes #4646
closes #4645

Impact assessment by core usage search and poggit suggests that the impact of this change will be close to zero.
However, since it changes behaviour which plugins might be unknowingly relying on, it's going into 4.1 rather than a patch release.

src/utils/Random.php

@@ -94,7 +94,7 @@ class Random{
 		$this->z = $this->w;
 		$this->w = ($this->w ^ (($this->w >> 19) & 0x7fffffff) ^ ($t ^ (($t >> 8) & 0x7fffffff))) & 0xffffffff;
 
-		return $this->w;
+		return Binary::signInt($this->w);
 	}
 
 	/**

tests/phpunit/utils/RandomTest.php

@@ -0,0 +1,43 @@
+<?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\utils;
+
+use PHPUnit\Framework\TestCase;
+
+class RandomTest extends TestCase{
+
+	public function testNextSignedIntReturnsSignedInts() : void{
+		//use a known seed which should definitely produce negatives
+		$random = new Random(0);
+		$negatives = false;
+
+		for($i = 0; $i < 100; ++$i){
+			if($random->nextSignedInt() < 0){
+				$negatives = true;
+				break;
+			}
+		}
+		self::assertTrue($negatives);
+	}
+}