Time machine mode [Z#23129725] (#3961)

Allows organizers to test their shop as if it were a different date and time.

Implemented using a time_machine_now() function which is used instead of regular now(), which can overlay the real date time with a value from a ContextVar, assigned from a session value in EventMiddleware.

For more information, see doc/development/implementation/timemachine.rst

---------

Co-authored-by: Richard Schreiber <schreiber@rami.io>
Co-authored-by: Raphael Michel <michel@rami.io>

src/tests/presale/test_timemachine.py

@@ -0,0 +1,43 @@
+#
+# This file is part of pretix (Community Edition).
+#
+# Copyright (C) 2014-2020 Raphael Michel and contributors
+# Copyright (C) 2020-2021 rami.io GmbH and contributors
+#
+# This program is free software: you can redistribute it and/or modify it under the terms of the GNU Affero General
+# Public License as published by the Free Software Foundation in version 3 of the License.
+#
+# ADDITIONAL TERMS APPLY: Pursuant to Section 7 of the GNU Affero General Public License, additional terms are
+# applicable granting you additional permissions and placing additional restrictions on your usage of this software.
+# Please refer to the pretix LICENSE file to obtain the full terms applicable to this work. If you did not receive
+# this file, see <https://pretix.eu/about/en/license>.
+#
+# This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied
+# warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along with this program.  If not, see
+# <https://www.gnu.org/licenses/>.
+#
+from django_scopes.state import scopes_disabled
+
+from pretix.base.models import Team, User
+
+
+class TimemachineTestMixin:
+    @scopes_disabled()
+    def _login_with_permission(self, orga):
+        self.user = User.objects.create_user('dummy@dummy.dummy', 'dummy')
+        self.team1 = Team.objects.create(organizer=orga, can_create_events=True, can_change_event_settings=True,
+                                         can_change_items=True, all_events=True)
+        self.team1.members.add(self.user)
+        self.client.login(email='dummy@dummy.dummy', password='dummy')
+
+    def _set_time_machine_now(self, dt):
+        session = self.client.session
+        session[f'timemachine_now_dt:{self.event.pk}'] = str(dt)
+        session.save()
+
+    def _enable_test_mode(self):
+        self.event.testmode = True
+        self.event.save()