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/tests/presale/test_checkout.py

@@ -1799,3 +1799,35 @@ class CheckoutTestCase(TestCase):
         response = self.client.post('/%s/%s/checkout/confirm/' % (self.orga.slug, self.event.slug), follow=True)
         doc = BeautifulSoup(response.rendered_content, "lxml")
         self.assertEqual(len(doc.select(".thank-you")), 1)
+
+    def test_create_testmode_order_in_testmode(self):
+        self.event.testmode = True
+        self.event.save()
+        CartPosition.objects.create(
+            event=self.event, cart_id=self.session_key, item=self.ticket,
+            price=23, expires=now() + timedelta(minutes=10)
+        )
+        self._set_session('payment', 'banktransfer')
+
+        response = self.client.get('/%s/%s/checkout/confirm/' % (self.orga.slug, self.event.slug), follow=True)
+        assert "test mode" in response.rendered_content
+        response = self.client.post('/%s/%s/checkout/confirm/' % (self.orga.slug, self.event.slug), follow=True)
+        doc = BeautifulSoup(response.rendered_content, "lxml")
+        self.assertEqual(len(doc.select(".thank-you")), 1)
+        assert Order.objects.last().testmode
+        assert Order.objects.last().code[1] == "0"
+
+    def test_do_not_create_testmode_order_without_testmode(self):
+        CartPosition.objects.create(
+            event=self.event, cart_id=self.session_key, item=self.ticket,
+            price=23, expires=now() + timedelta(minutes=10)
+        )
+        self._set_session('payment', 'banktransfer')
+
+        response = self.client.get('/%s/%s/checkout/confirm/' % (self.orga.slug, self.event.slug), follow=True)
+        assert "test mode" not in response.rendered_content
+        response = self.client.post('/%s/%s/checkout/confirm/' % (self.orga.slug, self.event.slug), follow=True)
+        doc = BeautifulSoup(response.rendered_content, "lxml")
+        self.assertEqual(len(doc.select(".thank-you")), 1)
+        assert not Order.objects.last().testmode
+        assert "0" not in Order.objects.last().code