Remove forced type mixed on PHPUnit test to be compliant with PHP 7.4.

Author: niconoe-

tests/PHPStan/Analyser/nsrt/filter-var-returns-non-empty-string.php

@@ -26,7 +26,7 @@ public function run(
 		int $positive_int,
 		int $negative_int,
 		bool $bool,
-		mixed $mixed,
+		$mixed,
 	): void
 	{
 		$array = [];