tests: add helper and some initial docs about testing

docs/tests.md

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+# Tests
+
+## Backend Tests
+
+We use pytest in combination with pytest-django and factoryboy to write backend
+tests.
+
+### Notes
+
+#### Testing Emails
+
+The django email backend does not guarantee a deterministic order of the mails,
+so checking against an index will result in flaky tests. If you want to assert
+that a mail was sent use one of the helper functions in `tests/helpers.py`.

tests/helpers.py

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+from django.core import mail
+
+
+def get_emails_for_address(email_address):
+    """Return all mails send to email_address"""
+    index = list(filter(lambda mails: mails.to[0] == email_address, mail.outbox))
+    return index