Add couple of test for AuthToken (#4816)

* Add couple of test for AuthToken

* PEP8 cleanup

tests/test_authtoken.py

+import pytest
+from django.contrib.admin import site
+from django.contrib.auth.models import User
+from django.test import TestCase
+
+from rest_framework.authtoken.admin import TokenAdmin
+from rest_framework.authtoken.models import Token
+from rest_framework.authtoken.serializers import AuthTokenSerializer
+from rest_framework.exceptions import ValidationError
+
+
+class AuthTokenTests(TestCase):
+
+    def setUp(self):
+        self.site = site
+        self.user = User.objects.create_user(username='test_user')
+        self.token = Token.objects.create(key='test token', user=self.user)
+
+    def test_model_admin_displayed_fields(self):
+        mock_request = object()
+        token_admin = TokenAdmin(self.token, self.site)
+        assert token_admin.get_fields(mock_request) == ('user',)
+
+    def test_token_string_representation(self):
+        assert str(self.token) == 'test token'
+
+    def test_validate_raise_error_if_no_credentials_provided(self):
+        with pytest.raises(ValidationError):
+            AuthTokenSerializer().validate({})