💚 Force deterministic ordering to avoid test flakiness

Author: sergei-maertens

src/openforms/submissions/models/submission.py

@@ -714,7 +714,7 @@ def get_co_signer(self) -> str:
     def get_attachments(self) -> SubmissionFileAttachmentQuerySet:
         from .submission_files import SubmissionFileAttachment
 
-        return SubmissionFileAttachment.objects.for_submission(self)
+        return SubmissionFileAttachment.objects.for_submission(self).order_by("pk")
 
     @property
     def attachments(self) -> SubmissionFileAttachmentQuerySet: