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/base/test_invoices.py

@@ -291,18 +291,28 @@ def test_invoice_numbers(env):
     )
     order2.fees.create(fee_type=OrderFee.FEE_TYPE_PAYMENT, value=Decimal('0.25'), tax_rate=Decimal('0.00'),
                        tax_value=Decimal('0.00'))
+    testorder = Order.objects.create(
+        code='BAR', event=event, email='dummy2@dummy.test',
+        status=Order.STATUS_PENDING,
+        datetime=now(), expires=now() + timedelta(days=10),
+        total=0, testmode=True,
+        locale='en'
+    )
     inv1 = generate_invoice(order)
     inv2 = generate_invoice(order)
+    invt1 = generate_invoice(testorder)
 
     event.settings.set('invoice_numbers_consecutive', False)
     inv3 = generate_invoice(order)
     inv4 = generate_invoice(order)
     inv21 = generate_invoice(order2)
+    invt2 = generate_invoice(testorder)
     inv22 = generate_invoice(order2)
 
     event.settings.set('invoice_numbers_consecutive', True)
     inv5 = generate_invoice(order)
     inv6 = generate_invoice(order)
+    invt3 = generate_invoice(testorder)
     inv7 = generate_invoice(order)
     Invoice.objects.filter(pk=inv6.pk).delete()  # This should never ever happen, but what if it happens anyway?
     inv8 = generate_invoice(order)
@@ -328,6 +338,10 @@ def test_invoice_numbers(env):
     assert inv1.number == '{}-00001'.format(event.slug.upper())
     assert inv3.number == '{}-{}-3'.format(event.slug.upper(), order.code)
 
+    assert invt1.number == '{}-TEST-00001'.format(event.slug.upper())
+    assert invt2.number == '{}-TEST-{}-2'.format(event.slug.upper(), testorder.code)
+    assert invt3.number == '{}-TEST-00002'.format(event.slug.upper())
+
 
 @pytest.mark.django_db
 def test_invoice_number_prefixes(env):