Mocking Laravel custom validation rule class not working

I'm implementing a new custom validation rule in form submit. But I want to bypass the validation rule in unit testing. Below is the simplified of the validation rule and unit test class. What am I missing?

namespace App\Rules;

use Illuminate\Contracts\Validation\Rule;

class Captcha implements Rule
{
    public function passes($attribute, $value)
    {
        // assuming will always return false in testing
        // works fine when true
        return false; 
    }

    public function message()
    {
        return 'Captcha error! Try again later or contact site admin.';
    }
}

use Tests\TestCase;
use App\Rules\Captcha;

class RegistrationTest extends TestCase {

    public test_user_registration()
    {
        $this->mock(Captcha::class, function ($mock) {
            $mock->shouldReceive('passes')->andReturn(true);
        });

        $response = $this->post(route('tenant.register'), [
            'g-recaptcha-response' => 1,
            'email' => 'user@example.com',
            'password' => 'secret',
        ]);

        $this->assertEquals(1, User::all()->count());
     }
} 

via Chebli Mohamed