Clean up old Djrill/Mandrill tests

* Match other ESP test strategies for
  test_mandrill_backend and
  test_mandrill_integration

* Extract test_mandrill_session_sharing into
  SessionSharingTestCasesMixin for all
  requests-based ESP backends

* Move leftover Djrill feature tests into
  test_mandrill_djrill features (until they are
  handled as part of esp_extra or in normalized
  ESP template/merge features)

Closes #7

anymail/tests/test_postmark_backend.py

@@ -16,7 +16,7 @@ from anymail.exceptions import (AnymailAPIError, AnymailSerializationError,
                                 AnymailUnsupportedFeature, AnymailRecipientsRefused)
 from anymail.message import attach_inline_image_file
 
-from .mock_requests_backend import RequestsBackendMockAPITestCase
+from .mock_requests_backend import RequestsBackendMockAPITestCase, SessionSharingTestCasesMixin
 from .utils import sample_image_content, sample_image_path, SAMPLE_IMAGE_FILENAME, AnymailTestMixin, decode_att
 
 
@@ -517,6 +517,11 @@ class PostmarkBackendRecipientsRefusedTests(PostmarkBackendMockAPITestCase):
         self.assertEqual(status.recipients['spam@example.com'].status, 'rejected')
 
 
+class PostmarkBackendSessionSharingTestCase(SessionSharingTestCasesMixin, PostmarkBackendMockAPITestCase):
+    """Requests session sharing tests"""
+    pass  # tests are defined in the mixin
+
+
 @override_settings(ANYMAIL_SEND_DEFAULTS={
     'tags': ['globaltag'],
     'track_opens': True,