[EE] Fix a failure because we stub DiffHelper#diff_file_blob_raw_path instead of #diff_file_blob_raw_url

Rémy Coutablerequested to merge
ee-41291-spec-failure-on-spec-features-merge_requests-image_diff_notes_spec-rb into master

EE MR for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16052.

What does this MR do?

Fix a failure because we stub DiffHelper#diff_file_blob_raw_path instead of #diff_file_blob_raw_url.

Edited by Rémy Coutable

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