Add some unit testing for the SalesOrder model

InvenTree/order/migrations/0032_auto_20200427_0044.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.0.5 on 2020-04-27 00:44
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('part', '0035_auto_20200406_0045'),
+        ('order', '0031_auto_20200426_0612'),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name='salesorderlineitem',
+            unique_together={('order', 'part')},
+        ),
+    ]