ext/gd: fix gh16559 and gh17349 tests

ext/gd/tests/gh16559.phpt: skip when using an external libgd
This is a test for a bugfix in the bundled libgd.

ext/gd/tests/gh17349.phpt: require PNG support
This test loads a PNG image, so if an external libgd is being used, it
has to support PNG.

close GH-21987

Author: orlitzky

ext/gd/tests/gh16559.phpt

@@ -2,6 +2,13 @@
 GH-16559 (UBSan abort in ext/gd/libgd/gd_interpolation.c:1007)
 --EXTENSIONS--
 gd
+--SKIPIF--
+<?php
+    if (!GD_BUNDLED) {
+        /* external libgd bugs are not our problem */
+    	die("skip meaningful only for bundled libgd\n");
+    }
+?>
 --FILE--
 <?php
 $input = imagecreatefrompng(__DIR__ . '/gh10614.png');

ext/gd/tests/gh17349.phpt

@@ -2,6 +2,12 @@
 GH-17349 (Tiled truecolor filling looses single color transparency)
 --EXTENSIONS--
 gd
+--SKIPIF--
+<?php
+    if (!(imagetypes() & IMG_PNG)) {
+        die("skip No PNG support");
+    }
+?>
 --FILE--
 <?php
 require_once __DIR__ . "/func.inc";