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/plugins/pretixdroid/templates/pretixplugins/pretixdroid/configuration_code.html

@@ -63,6 +63,11 @@
         </p>
         <h2>{% trans "3. Start scanning tickets" %}</h2>
         <script type="text/javascript" src="{% static "pretixplugins/pretixdroid/pretixdroid.js" %}"></script>
+        {% if request.event.testmode %}
+            <div class="alert-info">
+                {% trans "Test mode orders will only be scanned if you scan online. If you scan in asynchronous mode, test mode orders won't be there." %}
+            </div>
+        {% endif %}
     {% endif %}
 {% endblock %}
 

src/pretix/plugins/pretixdroid/views.py

@@ -332,6 +332,7 @@ class ApiDownloadView(ApiView):
             order__event=self.event,
             order__status__in=[Order.STATUS_PAID] + ([Order.STATUS_PENDING] if self.config.list.include_pending else
                                                      []),
+            order__testmode=False,
             subevent=self.config.list.subevent
         ).annotate(
             last_checked_in=Subquery(cqs)