Add test case for abstract classes

tests/classes/class_abstract.phpt

@@ -0,0 +1,28 @@
+--TEST--
+An abstract class cannot be instanciated
+--SKIPIF--
+<?php if (version_compare(zend_version(), '2.0.0-dev', '<')) die('skip ZendEngine 2 needed'); ?>
+--FILE--
+<?php
+
+abstract class base {
+	function show() {
+		echo "base\n";
+	}
+}
+
+class derived extends base {
+}
+
+$t = new derived();
+$t->show();
+
+$t = new base();
+$t->show();
+
+echo "Done\n"; // shouldn't be displayed
+?>
+--EXPECTF--
+base
+
+Fatal error: Cannot instantiate abstract class base in %s on line %d