* add phpunit as a dev dependency

* add some basic tests for UrlHelper::rewrite_relative()
 * fix UrlHelper::rewrite_relative() to work better on non-absolute
   relative URL paths

tests/UrlHelperTest.php

@@ -0,0 +1,54 @@
+<?php
+set_include_path(dirname(__DIR__) ."/include" . PATH_SEPARATOR .
+		get_include_path());
+
+require_once "autoload.php";
+require_once "functions.php";
+
+use PHPUnit\Framework\TestCase;
+
+final class UrlHelperTest extends TestCase {
+	public function testCanBeUsedAsString(): void {
+		/*$this->assertEquals(
+			'http://example.com/example.html',
+			UrlHelper::rewrite_relative('http://example.com/example/', '/example.html')
+		);
+
+		$this->assertEquals(
+			'http://example.com/example/example.html',
+			UrlHelper::rewrite_relative('http://example.com/example/', 'example.html')
+		);*/
+
+		// protocol-neutral URL
+		$this->assertEquals(
+			'https://example.com/example.html',
+			UrlHelper::rewrite_relative('http://example.com/example/', '//example.com/example.html')
+		);
+
+		// magnet allowed because it's a href attribute
+		$this->assertEquals(
+			'magnet:?xt=urn:btih:...',
+			UrlHelper::rewrite_relative('http://example.com/example/',
+				'magnet:?xt=urn:btih:...',
+				"a", "href", "")
+		);
+
+		// disallowed magnet
+		$this->assertEquals(
+			'http://example.com?xt=urn:btih:...',
+			UrlHelper::rewrite_relative('http://example.com/example/',
+				'magnet:?xt=urn:btih:...')
+		);
+
+		$this->assertEquals(
+			'https://apod.nasa.gov/apod/image/2203/Road2Stars_EsoHoralek_1080.jpg',
+			UrlHelper::rewrite_relative('https://apod.nasa.gov/apod/ap220315.html', 'image/2203/Road2Stars_EsoHoralek_1080.jpg')
+		);
+
+		$this->assertEquals(
+			'https://apod.nasa.gov/apod/image/2203/Road2Stars_EsoHoralek_1080.jpg',
+			UrlHelper::rewrite_relative('https://apod.nasa.gov/apod/ap220315.html', './image/2203/Road2Stars_EsoHoralek_1080.jpg')
+		);
+
+	}
+}