ext/gmp: Add test about manually (un)serializing

ext/gmp/tests/serialize_via_methods.phpt

@@ -0,0 +1,35 @@
+--TEST--
+GMP serialization and unserialization via magic methods
+--EXTENSIONS--
+gmp
+--FILE--
+<?php
+
+$n = gmp_init(13);
+var_dump($n);
+$a = $n->__serialize();
+var_dump($a);
+
+$n -= 11;
+var_dump($n);
+$n->__unserialize($a);
+var_dump($n);
+
+?>
+--EXPECT--
+object(GMP)#1 (1) {
+  ["num"]=>
+  string(2) "13"
+}
+array(1) {
+  [0]=>
+  string(1) "d"
+}
+object(GMP)#2 (1) {
+  ["num"]=>
+  string(1) "2"
+}
+object(GMP)#2 (1) {
+  ["num"]=>
+  string(2) "13"
+}