如何将会话密钥从控制器传递到laravel中的功能测试？

Registration Controller

session()->put('docmob',$request->_mobile);
$request->session()->keep(['docmob']); 

RegisterTest

public function test_Register()
{
//   $request=new Request();
// Session::start();
// $request->session()->keep(['docmob', '9871983585']);
//    dd(session('docmob'));
    $this->startSession();
    $this->post(route('doc-final-registration',[
   'f_name'=>'',
'l_name' => 'Jain',
'email'=>'vaibhavj1994@gmail.com',
'_password' => '1234567',
'_cpassword'=>'1234567',
'_token' => csrf_token(),
]) )->assertSessionHas('docmob');
}

And it gives not expected Output but give error:

1) Tests\Feature\FinalRegTest::test_Register

Session is missing expected key [docmob].

Failed asserting that false is true.

So, My question is to pass session key from controller to feature testing?? Please help me to find to solve it.