Split model_fields tests into different files.

tests/model_fields/test_manytomanyfield.py

@@ -0,0 +1,58 @@
+from django.apps import apps
+from django.db import models
+from django.test import SimpleTestCase
+from django.test.utils import isolate_apps
+
+
+class ManyToManyFieldTests(SimpleTestCase):
+
+    def test_abstract_model_pending_operations(self):
+        """
+        Many-to-many fields declared on abstract models should not add lazy
+        relations to resolve relationship declared as string (#24215).
+        """
+        pending_ops_before = list(apps._pending_operations.items())
+
+        class AbstractManyToManyModel(models.Model):
+            fk = models.ForeignKey('missing.FK', models.CASCADE)
+
+            class Meta:
+                abstract = True
+
+        self.assertIs(AbstractManyToManyModel._meta.apps, apps)
+        self.assertEqual(
+            pending_ops_before,
+            list(apps._pending_operations.items()),
+            'Pending lookup added for a many-to-many field on an abstract model'
+        )
+
+    @isolate_apps('model_fields', 'model_fields.tests')
+    def test_abstract_model_app_relative_foreign_key(self):
+        class AbstractReferent(models.Model):
+            reference = models.ManyToManyField('Refered', through='Through')
+
+            class Meta:
+                app_label = 'model_fields'
+                abstract = True
+
+        def assert_app_model_resolved(label):
+            class Refered(models.Model):
+                class Meta:
+                    app_label = label
+
+            class Through(models.Model):
+                refered = models.ForeignKey('Refered', on_delete=models.CASCADE)
+                referent = models.ForeignKey('ConcreteReferent', on_delete=models.CASCADE)
+
+                class Meta:
+                    app_label = label
+
+            class ConcreteReferent(AbstractReferent):
+                class Meta:
+                    app_label = label
+
+            self.assertEqual(ConcreteReferent._meta.get_field('reference').related_model, Refered)
+            self.assertEqual(ConcreteReferent.reference.through, Through)
+
+        assert_app_model_resolved('model_fields')
+        assert_app_model_resolved('tests')