Refactor the test structure

This renames the tests.sandbox app to tests.site to clear up some
confusion with the two sandbox app's.

tests/conftest.py

@@ -1,7 +1,6 @@
 from __future__ import absolute_import, unicode_literals
 
 import pytest
-from wagtail.wagtailcore.models import Page, Site
 
 pytest_plugins = [
     'tests.fixtures'
@@ -10,7 +9,9 @@ pytest_plugins = [
 
 @pytest.fixture(scope='session')
 def django_db_setup(django_db_setup, django_db_blocker):
+    from wagtail.wagtailcore.models import Page, Site
+
     with django_db_blocker.unblock():
-        # Remove some initial data that is brought by the sandbox module
+        # Remove some initial data that is brought by the tests.site module
         Site.objects.all().delete()
         Page.objects.all().exclude(depth=1).delete()