Move ImportingModelSerializerTests and add comments.

rest_framework/tests/test_serializer_import.py

+from django.test import TestCase
+
+from rest_framework import serializers
+from rest_framework.tests.accounts.serializers import AccountSerializer
+
+
+class ImportingModelSerializerTests(TestCase):
+    """
+    In some situations like, GH #1225, it is possible, especially in
+    testing, to import a serializer who's related models have not yet
+    been resolved by Django. `AccountSerializer` is an example of such
+    a serializer (imported at the top of this file).
+    """
+    def test_import_model_serializer(self):
+        """
+        The serializer at the top of this file should have been
+        imported successfully, and we should be able to instantiate it.
+        """
+        self.assertIsInstance(AccountSerializer(), serializers.ModelSerializer)

rest_framework/tests/test_serializer_nested.py

@@ -6,7 +6,6 @@ Doesn't cover model serializers.
 from __future__ import unicode_literals
 from django.test import TestCase
 from rest_framework import serializers
-from rest_framework.tests.accounts.serializers import AccountSerializer
 from . import models
 
 
@@ -346,8 +345,3 @@ class NestedModelSerializerUpdateTests(TestCase):
         result = deserialize.object
         result.save()
         self.assertEqual(result.id, john.id)
-
-
-class ImportingModelSerializerWithStrForeignKeys(TestCase):
-    def test_import_model_serializer(self):
-        self.assertIsInstance(AccountSerializer(), serializers.ModelSerializer)