Unit Test Laravel's FormRequest

I am trying to unit test various custom FormRequest objects. I found solutions that:

1. Suggest using the $this->call(…) method and assert the response with the expected value (link to answer). This is overkill, because it creates a direct dependency on Routing and Controllers.
2. Taylor’s test, found in laravel/framework/tests/Foundation/FoundationFormRequestTest.php. There is a lot of mocking and overhead done there.

I am looking for a solution where I can unit test individual fields (independent of other fields in the same request).

Sample FormRequest:

public function rules()
{
    return [
        'first_name' => 'required|between:2,50|alpha',
        'last_name' => 'required|between:2,50|alpha',
        'email'         => 'required|email|unique:users,email',
        'username'  => 'required|between:6,50|alpha_num|unique:users,username',
        'password'  => 'required|between:8,50|alpha_num|confirmed',
    ];
}

Desired Test:

public function testFirstNameField()
{
   // assertFalse, required
      ...
   // assertTrue, required
      ...
   // assertFalse, between
      ...
}

public function testLastNameField()
{
    ...
}

How can I unit test (assert) each validation rule of every field in isolation and individually?