billing: Add randomness to idempotency key while testing.

Makes it easier to test or regenerate fixtures for
test_billing_quantity_changes_end_to_end with generate=True.

corporate/lib/stripe.py

@@ -356,6 +356,8 @@ def process_billing_log_entry(processor: BillingProcessor, log_row: RealmAuditLo
     customer = Customer.objects.get(realm=log_row.realm)
     timestamp = datetime_to_timestamp(log_row.event_time)
     idempotency_key = 'process_billing_log_entry:%s' % (log_row.id,)
+    if settings.TEST_SUITE:
+        idempotency_key += '+' + generate_random_token(10)
     extra_args = {}  # type: Dict[str, Any]
     if log_row.extra_data is not None:
         extra_args = ujson.loads(log_row.extra_data)

corporate/tests/test_stripe.py

@@ -795,7 +795,8 @@ class StripeTest(ZulipTestCase):
                 self.assertEqual(subscription.quantity, quantity)
                 log_row = RealmAuditLog.objects.filter(
                     event_type=event_type, requires_billing_update=True).order_by('-id').first()
-                self.assertEqual(idempotency_key, 'process_billing_log_entry:%s' % (log_row.id,))
+                self.assertEqual(idempotency_key.split('+')[0],
+                                 'process_billing_log_entry:%s' % (log_row.id,))
                 self.assertEqual(subscription.proration_date, datetime_to_timestamp(log_row.event_time))
             with patch.object(stripe.Subscription, 'save', autospec=True,
                               side_effect=check_subscription_save):