Add test to verify file_get_contents error with folder

Closes GH-6600.

Author: afilina

ext/standard/tests/file/file_get_contents_error_folder-win.phpt

@@ -0,0 +1,12 @@
+--TEST--
+Test file_get_contents() function : error when passing folder - on Windows
+--SKIPIF--
+<?php
+if (substr(PHP_OS, 0, 3) != "WIN") { print "skip - Windows only"; }
+?>
+--FILE--
+<?php
+file_get_contents(__DIR__);
+?>
+--EXPECTF--
+Warning: file_get_contents(%s): Failed to open stream: Permission denied in %s on line %d

ext/standard/tests/file/file_get_contents_error_folder.phpt

@@ -0,0 +1,12 @@
+--TEST--
+Test file_get_contents() function : error when passing folder
+--SKIPIF--
+<?php
+if (substr(PHP_OS, 0, 3) === "WIN") { print "skip - not valid for Windows"; }
+?>
+--FILE--
+<?php
+file_get_contents(__DIR__);
+?>
+--EXPECTF--
+Notice: file_get_contents(): Read of %d bytes failed with errno=21 Is a directory in %s on line %d