Add some unit tests

- Saving a model automatically updates the reference_int field
- Data migrations are correctly applied

InvenTree/build/migrations/0032_auto_20211014_0632.py

@@ -10,12 +10,8 @@ def build_refs(apps, schema_editor):
     Rebuild the integer "reference fields" for existing Build objects
     """
 
-    print("\n - Rebuilding reference field for BuildOrder model...")
-
     BuildOrder = apps.get_model('build', 'build')
 
-    n = BuildOrder.objects.count()
-
     for build in BuildOrder.objects.all():
 
         ref = 0
@@ -31,9 +27,6 @@ def build_refs(apps, schema_editor):
         build.reference_int = ref
         build.save()
 
-    print(f" - Updated {n} BuildOrder objects")
-    print(f" - COMPLETE! -")
-
 def unbuild_refs(apps, schema_editor):
     """
     Provided only for reverse migration compatibility