email_backends: Fix concurrent backend testing for generate_emails.

Previously, this command would reliably fail:

```
tools/test-backend --skip-provision-check --parallel=3
  zerver.tests.test_email_log.EmailLogTest.test_forward_address_details
  zerver.tests.test_email_log.EmailLogTest.test_generate_and_clear_email_log
  zerver.tests.test_example.TestDevelopmentEmailsLog
```

and now it reliably succeeds. :-)

After hours of fiddling/googling/hair-tearing, I found that
mocking-away Django Connection.send_messages() was the best:

- We're testing Zulip and not Django.
- Mocking at this lower level exercises more of our code.
- EmailLogBackEnd._do_send_messages() helper method added to simplify mocking.

Fixes #21925.

zerver/tests/test_email_log.py

@@ -10,7 +10,7 @@ from zproject.email_backends import get_forward_address
 class EmailLogTest(ZulipTestCase):
     def test_generate_and_clear_email_log(self) -> None:
         with self.settings(EMAIL_BACKEND="zproject.email_backends.EmailLogBackEnd"), mock.patch(
-            "zproject.email_backends.EmailBackend.send_messages"
+            "zproject.email_backends.EmailLogBackEnd._do_send_messages", lambda *args: 1
         ), self.assertLogs(level="INFO") as m, self.settings(DEVELOPMENT_LOG_EMAILS=True):
             result = self.client_get("/emails/generate/")
             self.assertEqual(result.status_code, 302)
@@ -35,8 +35,9 @@ class EmailLogTest(ZulipTestCase):
 
         self.assertEqual(get_forward_address(), forward_address)
 
-        with self.settings(EMAIL_BACKEND="zproject.email_backends.EmailLogBackEnd"):
-            with mock.patch("zproject.email_backends.EmailBackend.send_messages"):
+        with self.settings(EMAIL_BACKEND="zproject.email_backends.EmailLogBackEnd"), mock.patch(
+            "zproject.email_backends.EmailLogBackEnd._do_send_messages", lambda *args: 1
+        ):
             result = self.client_get("/emails/generate/")
             self.assertEqual(result.status_code, 302)
             self.assertIn("emails", result["Location"])

zerver/tests/test_example.py

@@ -401,8 +401,9 @@ class TestDevelopmentEmailsLog(ZulipTestCase):
         # https://docs.python.org/3/library/unittest.html#unittest.TestCase.assertLogs
         with self.settings(EMAIL_BACKEND="zproject.email_backends.EmailLogBackEnd"), self.settings(
             DEVELOPMENT_LOG_EMAILS=True
-        ), self.assertLogs(level="INFO") as logger:
-
+        ), self.assertLogs(level="INFO") as logger, mock.patch(
+            "zproject.email_backends.EmailLogBackEnd._do_send_messages", lambda *args: 1
+        ):
             result = self.client_get(
                 "/emails/generate/"
             )  # Generates emails and redirects to /emails/

zproject/email_backends.py

@@ -82,11 +82,19 @@ class EmailLogBackEnd(EmailBackend):
 
             email_message.to = [get_forward_address()]
 
+    # This wrapper function exists to allow tests easily to mock the
+    # step of trying to send the emails. Previously, we had mocked
+    # Django's connection.send_messages(), which caused unexplained
+    # test failures when running test-backend at very high
+    # concurrency.
+    def _do_send_messages(self, email_messages: List[EmailMultiAlternatives]) -> int:
+        return super().send_messages(email_messages)  # nocoverage
+
     def send_messages(self, email_messages: List[EmailMultiAlternatives]) -> int:
         num_sent = len(email_messages)
         if get_forward_address():
             self.prepare_email_messages_for_forwarding(email_messages)
-            num_sent = super().send_messages(email_messages)
+            num_sent = self._do_send_messages(email_messages)
 
         if settings.DEVELOPMENT_LOG_EMAILS:
             for email in email_messages: