Fix tests not to use implicit cast to string

Author: JakeQZ

tests/Unit/Value/ExpressionTest.php

@@ -4,6 +4,7 @@
 
 use PHPUnit\Framework\TestCase;
 use Sabberworm\CSS\Parsing\ParserState;
+use Sabberworm\CSS\OutputFormat;
 use Sabberworm\CSS\Settings;
 use Sabberworm\CSS\Value\Value;
 use Sabberworm\CSS\Value\ValueList;
@@ -48,7 +49,7 @@ public function parseExpressions(string $input, string $expected, int $expressio
 
         self::assertInstanceOf(ValueList::class, $val);
         self::assertInstanceOf(Expression::class, $val->getListComponents()[$expression_index]);
-        self::assertSame($expected, (string) $val);
+        self::assertSame($expected, $val->render(OutputFormat::createCompact()));
     }
 
     private function getDelimiters(string $rule): array