Add an initial openssl decrypt GCM test

ext/openssl/tests/openssl_decrypt_gcm.phpt

@@ -0,0 +1,36 @@
+--TEST--
+openssl_decrypt() with GCM cipher algorithm tests
+--SKIPIF--
+<?php
+if (!extension_loaded("openssl"))
+	die("skip");
+if (!in_array('aes-128-gcm', openssl_get_cipher_methods()))
+	die("skip: aes-128-gcm not available");
+?>
+--FILE--
+<?php
+require_once __DIR__ . "/cipher_tests.inc";
+$method = 'aes-128-gcm';
+$tests = openssl_get_cipher_tests($method);
+
+foreach ($tests as $idx => $test) {
+	echo "TEST $idx\n";
+	$pt = openssl_decrypt($test['ct'], $method, $test['key'], OPENSSL_RAW_DATA,
+		$test['iv'], $test['tag'], $test['aad']);
+	var_dump($test['pt'] === $pt);
+}
+
+?>
+--EXPECT--
+TEST 0
+bool(true)
+TEST 1
+bool(true)
+TEST 2
+bool(true)
+TEST 3
+bool(true)
+TEST 4
+bool(true)
+TEST 5
+bool(true)