Add test

Author: VincentLanglet

Diff for: tests/PHPStan/Analyser/nsrt/array-is-list-unset.php

@@ -23,4 +23,7 @@ function () {
 	$a[] = 4;
 	assertType('array{0: 1, 1: 2, 3: 4}', $a);
 	assertType('false', array_is_list($a));
+	unset($a[3]);
+	assertType('array{0: 1, 1: 2}', $a);
+	assertType('true', array_is_list($a));
 };