spec: fix Assignment#generate_comments_from_files spec failure
spec: fix Assignment#generate_comments_from_files spec failure

"should work for groups" sometimes fails when the group submission
attachment is created on a user instead of the group

fixes CNVS-25886

Change-Id: Ifb397923cfedf70531182a0c86559934e9966490
Reviewed-on: https://gerrit.instructure.com/68914
Tested-by: Jenkins
Reviewed-by: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
Product-Review: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
QA-Review: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>

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