php 单元测试 phpunit ，如何mock类调用自身的方法？

<code>class ZabbixSystem implements ApmSystemInterface      public function querySessionIdTest()    {        echo 'querySessionIdTest';        return $this->sessionidtest();    }    public function sessionidtest()    {        echo 'sessionidtest';        return 'aaa';    }}</code>

<code>public function testQuerySessionIdSuccess()    {        $return = 'aaa';        $stub = Mockery::mock('mcokname');//        $this->app->instance('ZabbixSystem', $stub);        $stub->shouldReceive('querySessionIdTest')->andReturn($return);        $response = $stub->querySessionIdTest();        $this->assertEquals('aaa', $response);    }    public function testQuerySession()    {        // 为 SomeClass 类创建桩件。        $stub = $this->getMockBuilder('App\Services\ZabbixSystem')->setMethods(array(            'querySessionIdTest',            'sessionidtest'        ))->getMock();        // 配置桩件。        $stub->method('sessionidtest')->willReturn('111');        $this->app->instance('ZabbixSystem', $stub);        $this->assertEquals('foo', $stub->querySessionIdTest());    }</code>