Add flag testmode_supported to sales channels (#1455)

* Add testmode-support-flag to SalesChannels

* Make saleschannels/testmode-warnings even more dangerous!

* Add warning for payment-methods that do support testmode but are being used in a non-testmode order caused by a saleschannel in a testmode-shop.

* Remove redundant testmode_supported-flag for WebshopSalesChannel

* Raise error on API when sales_channel does not support testmode

* Tests

* Fix style issue after merge

src/tests/presale/test_cart.py

@@ -9,6 +9,7 @@ from django.utils.timezone import now
 from django_countries.fields import Country
 from django_scopes import scopes_disabled
 
+from pretix.base.channels import SalesChannel
 from pretix.base.decimal import round_decimal
 from pretix.base.models import (
     CartPosition, Event, InvoiceAddress, Item, ItemCategory, ItemVariation,
@@ -24,6 +25,13 @@ from pretix.testutils.scope import classscope
 from pretix.testutils.sessions import get_cart_session_key
 
 
+class FoobarSalesChannel(SalesChannel):
+    identifier = "bar"
+    verbose_name = "Foobar"
+    icon = "home"
+    testmode_supported = True
+
+
 class CartTestMixin:
     @scopes_disabled()
     def setUp(self):
@@ -759,7 +767,7 @@ class CartTest(CartTestMixin, TestCase):
         self.ticket.save()
         self.client.post('/%s/%s/cart/add' % (self.orga.slug, self.event.slug), {
             'item_%d' % self.ticket.id: '1',
-        }, follow=True, PRETIX_SALES_CHANNEL='bar')
+        }, follow=True, PRETIX_SALES_CHANNEL=FoobarSalesChannel)
         with scopes_disabled():
             self.assertEqual(CartPosition.objects.filter(cart_id=self.session_key, event=self.event).count(), 1)