Update test for changed behaviour of GMP constructor

Closed GH-10160

Signed-off-by: George Peter Banyard <girgias@php.net>

Author: nielsdos

ext/gmp/tests/gmp_dynamic_property.phpt

@@ -5,7 +5,7 @@ gmp
 --FILE--
 <?php
 
-$g = new GMP();
+$g = gmp_init(0);
 $g->{1} = 123;
 
 $serialized = serialize($g);