Should I be unit testing every piece of code

I recently started unit testing and I wonder if block tests should be written to cover 100% of the code?

It seems useless when I finish writing more unit testing code than production code.

I am writing a PHP Codeigniter project, and sometimes it seems like I am writing so much code to test one small function.

For an example it is Unit test

public function testLogin(){
    //setup
    $this->CI->load->library("form_validation");
    $this->realFormValidation=new $this->CI->form_validation;
    $this->CI->form_validation=$this->getMock("CI_Form_validation");
    $this->realAuth=new $this->CI->auth;
    $this->CI->auth=$this->getMock("Auth",array("logIn"));
    $this->CI->auth->expects($this->once())
                   ->method("logIn")
                   ->will($this->returnValue(TRUE));

    //test
    $this->CI->form_validation->expects($this->once())
        ->method("run")
        ->will($this->returnValue(TRUE));
    $_POST["login"]=TRUE;
    $this->CI->login();
    $out = $this->CI->output->get_headers();
    //check new header ends with dashboard
    $this->assertStringEndsWith("dashboard",$out[0][0]);

    //tear down
    $this->CI->form_validation=$this->realFormValidation;
    $this->CI->auth=$this->realAuth;

}
public function badLoginProvider(){
    return array(
        array(FALSE,FALSE),
        array(TRUE,FALSE)
    );
}
/**
 * @dataProvider badLoginProvider
 */
public function testBadLogin($formSubmitted,$validationResult){
    //setup
    $this->CI->load->library("form_validation");
    $this->realFormValidation=new $this->CI->form_validation;
    $this->CI->form_validation=$this->getMock("CI_Form_validation");

    //test
    $this->CI->form_validation->expects($this->any())
        ->method("run")
        ->will($this->returnValue($validationResult));
    $_POST["login"]=$formSubmitted;
    $this->CI->login();
    //check it went to the login page
    $out = output();
    $this->assertGreaterThan(0, preg_match('/Login/i', $out));

    //tear down
    $this->CI->form_validation=$this->realFormValidation;
}

For this production code

public function login(){
    if($this->input->post("login")){
        $this->load->library('form_validation');
        $username=$this->input->post('username');
        $this->form_validation->set_rules('username', 'Username', 'required');
        $this->form_validation->set_rules('password', 'Password', "required|callback_userPassCheck[$username]");
        if ($this->form_validation->run()===FALSE) {
            $this->load->helper("form");
            $this->load->view('dashboard/login');
        }
        else{
            $this->load->model('auth');
            echo "valid";
            $this->auth->logIn($this->input->post('username'),$this->input->post('password'),$this->input->post('remember_me'));
            $this->load->helper('url');
            redirect('dashboard');
        }
    }
    else{
        $this->load->helper("form");
        $this->load->view('dashboard/login');
    }
}

Where am I so wrong?