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_event.py

@@ -12,6 +12,7 @@ from django_scopes import scopes_disabled
 from pytz import timezone
 from tests.base import SoupTest
 
+from pretix.base.channels import SalesChannel
 from pretix.base.models import (
     Event, Item, ItemCategory, ItemVariation, Order, Organizer, Quota, Team,
     User, WaitingListEntry,
@@ -19,6 +20,13 @@ from pretix.base.models import (
 from pretix.base.models.items import SubEventItem, SubEventItemVariation
 
 
+class FoobarSalesChannel(SalesChannel):
+    identifier = "bar"
+    verbose_name = "Foobar"
+    icon = "home"
+    testmode_supported = True
+
+
 class EventTestMixin:
     @scopes_disabled()
     def setUp(self):
@@ -109,7 +117,7 @@ class ItemDisplayTest(EventTestMixin, SoupTest):
             q.items.add(item)
         html = self.client.get('/%s/%s/' % (self.orga.slug, self.event.slug)).rendered_content
         self.assertNotIn("Early-bird", html)
-        html = self.client.get('/%s/%s/' % (self.orga.slug, self.event.slug), PRETIX_SALES_CHANNEL="bar").rendered_content
+        html = self.client.get('/%s/%s/' % (self.orga.slug, self.event.slug), PRETIX_SALES_CHANNEL=FoobarSalesChannel).rendered_content
         self.assertIn("Early-bird", html)
 
     def test_timely_available(self):