Add a simple test mode (#1181)

- [x] Provide data model and configuration toggle
- [x] Allow to delete individual test orders
- [x] Add tests
- [x] Add a prominent warning message to the backend if test mode orders exist (even though test mode is off), as this leads to wrong statistics
- [x] Decide if and how to generate invoices for test orders as invoice numbers cannot be repeated or should not have gaps.
- [x] Decide if and how we expose test orders through the API, since our difference pull mechanism relies on the fact that orders cannot be deleted.
- [x] Decide if and how we want to couple test modes of payment providers?
- [ ] pretix.eu: Ignore test orders for billing
- [ ] Adjust payment providers: Mollie, bitpay, cash, fakepayment, sepadebit

![download](https://user-images.githubusercontent.com/64280/53009081-fe420d80-343a-11e9-8361-b8511c988598.png)

src/pretix/control/forms/filter.py

@@ -212,6 +212,7 @@ class EventOrderFilterForm(OrderFilterForm):
             ('overpaid', _('Overpaid')),
             ('underpaid', _('Underpaid')),
             ('pendingpaid', _('Pending (but fully paid)')),
+            ('testmode', _('Test mode')),
         ),
         required=False,
     )
@@ -298,6 +299,10 @@ class EventOrderFilterForm(OrderFilterForm):
                 status=Order.STATUS_PENDING,
                 require_approval=True
             )
+        elif fdata.get('status') == 'testmode':
+            qs = qs.filter(
+                testmode=True
+            )
         elif fdata.get('status') == 'cp':
             s = OrderPosition.objects.filter(
                 order=OuterRef('pk')