email_notifications: Complete test coverage for module.

Following the last commit, there was this WELCOME_EMAIL_SENDER code
without an existing automated test.

tools/test-backend

@@ -79,7 +79,6 @@ not_yet_fully_covered = [
     "zerver/lib/cache.py",
     "zerver/lib/cache_helpers.py",
     "zerver/lib/i18n.py",
-    "zerver/lib/email_notifications.py",
     "zerver/lib/send_email.py",
     "zerver/lib/url_preview/preview.py",
     "zerver/worker/queue_processors.py",

zerver/tests/test_email_notifications.py

@@ -1526,3 +1526,22 @@ class TestFollowupEmailDelay(ZulipTestCase):
         # Test date_joined == Friday
         user_profile.date_joined = datetime(2018, 1, 5, 1, 0, 0, 0, tzinfo=timezone.utc)
         self.assertEqual(followup_day2_email_delay(user_profile), timedelta(days=3, hours=-1))
+
+
+class TestCustomEmailSender(ZulipTestCase):
+    def test_custom_email_sender(self) -> None:
+        name = "Nonreg Email"
+        email = self.nonreg_email("test")
+        with override_settings(
+            WELCOME_EMAIL_SENDER={
+                "name": name,
+                "email": email,
+            }
+        ):
+            hamlet = self.example_user("hamlet")
+            enqueue_welcome_emails(hamlet)
+            scheduled_emails = ScheduledEmail.objects.filter(users=hamlet)
+            email_data = orjson.loads(scheduled_emails[0].data)
+            self.assertEqual(email_data["context"]["email"], self.example_email("hamlet"))
+            self.assertEqual(email_data["from_name"], name)
+            self.assertEqual(email_data["from_address"], email)