Use generic TestBackend for base functionality tests

* Create generic TestBackend that simply collects
  send parameters
* Change BackendSettingsTests to TestBackend,
  and add some missing cases
* Add UnsupportedFeatureTests
* Replace repetitive per-backend SEND_DEFAULTS
  test cases with single (and more comprehensive)
  SendDefaultsTests

anymail/backends/test.py

@@ -0,0 +1,121 @@
+from anymail.exceptions import AnymailAPIError
+from anymail.message import AnymailRecipientStatus
+
+from .base import AnymailBaseBackend, BasePayload
+from ..utils import get_anymail_setting
+
+
+class TestBackend(AnymailBaseBackend):
+    """
+    Anymail backend that doesn't do anything.
+
+    Used for testing Anymail common backend functionality.
+    """
+
+    def __init__(self, *args, **kwargs):
+        # Init options from Django settings
+        esp_name = self.esp_name
+        self.sample_setting = get_anymail_setting('sample_setting', esp_name=esp_name,
+                                                  kwargs=kwargs, allow_bare=True)
+        self.recorded_send_params = get_anymail_setting('recorded_send_params', default=[],
+                                                        esp_name=esp_name, kwargs=kwargs)
+        super(TestBackend, self).__init__(*args, **kwargs)
+
+    def build_message_payload(self, message, defaults):
+        return TestPayload(backend=self, message=message, defaults=defaults)
+
+    def post_to_esp(self, payload, message):
+        # Keep track of the send params (for test-case access)
+        self.recorded_send_params.append(payload.params)
+        try:
+            # Tests can supply their own message.test_response:
+            response = message.test_response
+            if isinstance(response, AnymailAPIError):
+                raise response
+        except AttributeError:
+            # Default is to return 'sent' for each recipient
+            status = AnymailRecipientStatus(message_id=1, status='sent')
+            response = {
+                'recipient_status': {email: status for email in payload.recipient_emails}
+            }
+        return response
+
+    def parse_recipient_status(self, response, payload, message):
+        try:
+            return response['recipient_status']
+        except KeyError:
+            raise AnymailAPIError('Unparsable test response')
+
+
+class TestPayload(BasePayload):
+    # For test purposes, just keep a dict of the params we've received.
+    # (This approach is also useful for native API backends -- think of
+    # payload.params as collecting kwargs for esp_native_api.send().)
+
+    def init_payload(self):
+        self.params = {}
+        self.recipient_emails = []
+
+    def set_from_email(self, email):
+        self.params['from'] = email
+
+    def set_to(self, emails):
+        self.params['to'] = emails
+        self.recipient_emails += [email.email for email in emails]
+
+    def set_cc(self, emails):
+        self.params['cc'] = emails
+        self.recipient_emails += [email.email for email in emails]
+
+    def set_bcc(self, emails):
+        self.params['bcc'] = emails
+        self.recipient_emails += [email.email for email in emails]
+
+    def set_subject(self, subject):
+        self.params['subject'] = subject
+
+    def set_reply_to(self, emails):
+        self.params['reply_to'] = emails
+
+    def set_extra_headers(self, headers):
+        self.params['extra_headers'] = headers
+
+    def set_text_body(self, body):
+        self.params['text_body'] = body
+
+    def set_html_body(self, body):
+        self.params['html_body'] = body
+
+    def add_alternative(self, content, mimetype):
+        self.unsupported_feature("alternative part with type '%s'" % mimetype)
+
+    def add_attachment(self, attachment):
+        self.params.setdefault('attachments', []).append(attachment)
+
+    def set_metadata(self, metadata):
+        self.params['metadata'] = metadata
+
+    def set_send_at(self, send_at):
+        self.params['send_at'] = send_at
+
+    def set_tags(self, tags):
+        self.params['tags'] = tags
+
+    def set_track_clicks(self, track_clicks):
+        self.params['track_clicks'] = track_clicks
+
+    def set_track_opens(self, track_opens):
+        self.params['track_opens'] = track_opens
+
+    def set_template_id(self, template_id):
+        self.params['template_id'] = template_id
+
+    def set_merge_data(self, merge_data):
+        self.params['merge_data'] = merge_data
+
+    def set_merge_global_data(self, merge_global_data):
+        self.params['merge_global_data'] = merge_global_data
+
+    def set_esp_extra(self, extra):
+        # Merge extra into params
+        self.params.update(extra)