ext/gd/tests: fix three misleading die() messages

Three of our gd tests could be skipped with a message about requiring
bundled GD, but those tests don't actually require bundled GD. We
update the messages to mention the specific functions that are
required.

Author: orlitzky

ext/gd/tests/bug24155.phpt

@@ -4,7 +4,7 @@ Bug #24155 (gdImageRotate270 rotation problem).
 gd
 --SKIPIF--
 <?php
-    if (!function_exists("imagerotate")) die("skip requires bundled GD library\n");
+    if (!function_exists("imagerotate")) die("skip requires imagerotate function");
 ?>
 --FILE--
 <?php

ext/gd/tests/bug39366.phpt

@@ -4,7 +4,7 @@ Bug #39366 (imagerotate does not respect alpha with angles>45)
 gd
 --SKIPIF--
 <?php
-    if (!function_exists("imagerotate")) die("skip requires bundled GD library\n");
+    if (!function_exists("imagerotate")) die("skip requires imagerotate function");
 ?>
 --FILE--
 <?php

ext/gd/tests/imagefilter.phpt

@@ -4,7 +4,7 @@ imagefilter() function test
 gd
 --SKIPIF--
 <?php
-    if (!function_exists("imagefilter")) die("skip requires bundled GD library\n");
+    if (!function_exists("imagefilter")) die("skip requires imagefilter function");
 ?>
 --FILE--
 <?php