Move common email tests to a mixin
Move common email tests to a mixin

* Create AllEmailsTestsMixin performing tests for all message types.
* Adapt HtmlEmailsTestCase and TextOnlyEmailsTestCase to inheritance
from AllEmailsTestsMixin.

ctxis/django-newsletter,dsanders11/django-newsletter,dsanders11/django-newsletter,dsanders11/django-newsletter,ctxis/django-newsletter,viaregio/django-newsletter,ctxis/django-newsletter,viaregio/django-newsletter