Export the document test mixins via the tests top level __init__

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/documents/tests/__init__.py

@@ -1,2 +1,3 @@
 from .base import GenericDocumentTestCase, GenericDocumentViewTestCase  # NOQA
 from .literals import *  # NOQA
+from .mixins import *  # NOQA

mayan/apps/documents/tests/mixins.py

@@ -8,6 +8,8 @@ from ..models import DocumentType
 
 from .literals import TEST_DOCUMENT_TYPE_LABEL, TEST_SMALL_DOCUMENT_FILENAME
 
+__all__ = ('DocumentTestMixin',)
+
 
 class DocumentTestMixin(object):
     auto_create_document_type = True