Issues with using guzzle in phpunit test.

My app is vue spa backed by Laravel Api. I am using Laravel passport for authentication. in login controller it receives the username and password from the request, and adds the 'passport_client_id' and passport_cleint_secret' to the $request, and uses Guzzle to make a request to the '/oauth/token>' url. This works fine. Then i proceeded to write a test to see if a user can login(i.e: that it receives a access_token). I am getting the following error

GuzzleHttp\Exception\ClientException: Client error: `POST http://localhost:8000/oauth/token` resulted in a `401 Unauthorized` response:
{"error":"invalid_client","error_description":"Client authentication failed","message":"Client authentication failed"}

I have checked if phpunit is missing any data, but it has all the data it needed. Interesting thing is if i make the same request from testclass, it does return a successful response.

My controller:

$http = new \GuzzleHttp\Client();
            $response = $http->post(config('services.passport.login_endpoint'), [
                'form_params' => [
                    'grant_type' => 'password',
                    'client_id' => config('services.passport.client_id'),
                    'client_secret' => config('services.passport.client_secret'),
                    'username' => $request->username,
                    'password' => $request->password,
                ]
            ]);
            return $response->getBody();

My test:

$this->artisan('passport:install');
        $passport_client_secret = DB::table('oauth_clients')->where('id',2)->value('secret');
        Config::set('services.passport.client_id', 2);
        Config::set('services.passport.client_secret', $passport_client_secret);
        
        $user = factory('App\User')->create();

        $response = $this->json('post','api/v1/login',[
            'username' => $user->email,
            'password' => 'password'
        ]);

For debugging , i tried directly making the api request from testclass using the following code , it succeeds.

$response = $this->json('post',config('services.passport.login_endpoint'), [
                'grant_type' => 'password',
                'client_id' => config('services.passport.client_id'),
                'client_secret' => config('services.passport.client_secret'),
                'username' => $user->email,
                'password' => 'password',
        ]);

Any idea why guzzle behaves different when used in PHPunit? What can i do to successfully test this controller? Thanks in advance.