Make this test portable

Author: sebastianbergmann

tests/unit/Util/FilesystemTest.php

@@ -9,6 +9,7 @@
  */
 namespace PHPUnit\Util;
 
+use const DIRECTORY_SEPARATOR;
 use PHPUnit\Framework\Attributes\CoversClass;
 use PHPUnit\Framework\Attributes\Small;
 use PHPUnit\Framework\TestCase;
@@ -22,7 +23,7 @@ public function testCanResolveStreamOrFile(): void
         $this->assertSame('php://stdout', Filesystem::resolveStreamOrFile('php://stdout'));
         $this->assertSame('socket://hostname:port', Filesystem::resolveStreamOrFile('socket://hostname:port'));
         $this->assertSame(__FILE__, Filesystem::resolveStreamOrFile(__FILE__));
-        $this->assertSame(__DIR__ . '/does-not-exist', Filesystem::resolveStreamOrFile(__DIR__ . '/does-not-exist'));
+        $this->assertSame(__DIR__ . DIRECTORY_SEPARATOR . 'does-not-exist', Filesystem::resolveStreamOrFile(__DIR__ . '/does-not-exist'));
         $this->assertFalse(Filesystem::resolveStreamOrFile(__DIR__ . '/does-not-exist/does-not-exist'));
     }
 }