zerver/tests: Require 100% test coverage of zerver/lib/upload.py.

The last line here is impossible to test.

Fixes #4489.

tools/test-backend

@@ -59,7 +59,6 @@ not_yet_fully_covered = {
     'zerver/lib/i18n.py',
     'zerver/lib/notifications.py',
     'zerver/lib/send_email.py',
-    'zerver/lib/upload.py',
     # Other lib files that ideally would coverage, but aren't sorted
     'zerver/__init__.py',
     'zerver/filters.py',

zerver/lib/upload.py

@@ -515,7 +515,7 @@ class LocalUploadBackend(ZulipUploadBackend):
 if settings.LOCAL_UPLOADS_DIR is not None:
     upload_backend = LocalUploadBackend()  # type: ZulipUploadBackend
 else:
-    upload_backend = S3UploadBackend()
+    upload_backend = S3UploadBackend()  # nocoverage
 
 def delete_message_image(path_id: str) -> bool:
     return upload_backend.delete_message_image(path_id)