test(lang): 添加多个语言特性相关的回归测试

- 添加 bug27443 测试：验证 defined() 函数返回正确的布尔类型
- 添加 bug27535 测试：验证循环引用对象不会导致崩溃问题
- 添加 bug29944 测试：验证 switch 语句中定义函数的功能正常
- 添加 bug30578 测试：验证析构函数调用时机与输出缓冲区处理

tests/core/lang/bug27443.phpt

@@ -0,0 +1,8 @@
+--TEST--
+Bug #27443 (defined() returns wrong type)
+--FILE--
+<?php
+echo gettype(defined('test'));
+?>
+--EXPECT--
+boolean

tests/core/lang/bug27535.phpt

@@ -0,0 +1,27 @@
+--TEST--
+Bug #27535 (Objects pointing to each other cause Apache to crash)
+--FILE--
+<?php
+
+class Class1
+{
+    public $_Class2_obj;
+}
+
+class Class2
+{
+    public $storage = '';
+
+    function __construct()
+    {
+        $this->storage = new Class1();
+
+        $this->storage->_Class2_obj = $this;
+    }
+}
+
+function main() {
+    $foo = new Class2();
+}
+?>
+--EXPECT--

tests/core/lang/bug29944.phpt

@@ -0,0 +1,20 @@
+--TEST--
+Bug #29944 (function defined in switch crashes PHP)
+--FILE--
+<?php
+function foo($bar) {
+    if (preg_match('/\d/', $bar)) return true;
+    return false;
+}
+
+function main() {
+    $a = 1;
+    $b = "1";
+    switch ($a) {
+        case 1:
+            echo foo($b);
+    }
+}
+?>
+--EXPECT--
+1

tests/core/lang/bug30578.phpt

@@ -0,0 +1,31 @@
+--TEST--
+Bug #30578 (Output buffers flushed before calling __destruct functions)
+--FILE--
+<?php
+
+class Example
+{
+    function __construct()
+    {
+        ob_start();
+        echo "This should be displayed last.\n";
+    }
+
+    function __destruct()
+    {
+        $buffered_data = ob_get_contents();
+        ob_end_clean();
+
+        echo "This should be displayed first.\n";
+        echo "Buffered data: $buffered_data";
+    }
+}
+
+function main() {
+    error_reporting(E_ALL);
+    $obj = new Example;
+}
+?>
+--EXPECT--
+This should be displayed first.
+Buffered data: This should be displayed last.