Removed model_package test dependency on contrib.sites

tests/model_package/models/article.py

-from django.contrib.sites.models import Site
 from django.db import models
 
 
+class Site(models.Model):
+    name = models.CharField(max_length=100)
+
+
 class Article(models.Model):
     sites = models.ManyToManyField(Site)
     headline = models.CharField(max_length=100)

tests/model_package/tests.py

 from __future__ import unicode_literals
 
-from django.contrib.sites.models import Site
 from django.db import models, connection
 from django.db.backends.utils import truncate_name
 from django.test import TestCase
 
 from .models.publication import Publication
-from .models.article import Article
+from .models.article import Article, Site
 
 
 class Advertisement(models.Model):
@@ -16,13 +15,6 @@ class Advertisement(models.Model):
 
 class ModelPackageTests(TestCase):
 
-    @classmethod
-    def setUpClass(cls):
-        super(ModelPackageTests, cls).setUpClass()
-        # This cleanup is necessary because contrib.sites cache
-        # makes tests interfere with each other, see #11505
-        Site.objects.clear_cache()
-
     def test_m2m_tables_in_subpackage_models(self):
         """
         Regression for #12168: models split into subpackages still get M2M
@@ -30,12 +22,11 @@ class ModelPackageTests(TestCase):
         """
         p = Publication.objects.create(title="FooBar")
 
-        current_site = Site.objects.get_current()
-        self.assertEqual(current_site.domain, "example.com")
+        site = Site.objects.create(name="example.com")
 
         a = Article.objects.create(headline="a foo headline")
         a.publications.add(p)
-        a.sites.add(current_site)
+        a.sites.add(site)
 
         a = Article.objects.get(id=a.pk)
         self.assertEqual(a.id, a.pk)