Add tests for the makemigrations override

tests/test_project/makemigrations_backward_incompatible_migration_missing/models.py

@@ -0,0 +1,6 @@
+from django.db import models
+
+
+class A(models.Model):
+    x = models.IntegerField()
+    new_field = models.CharField(null=False, default="empty", max_length=255)