swh-web: Add coverage to deposit admin page

That page is missing tests. We do not have to modify this often. Still recently, I had too and I felt quite naked.

I tried to add coverage [1] with the current tool but failed so far. I kept my latest tryout [2]

• [1] swh/devel/swh-web!319 (closed)

• [2] $673

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Migrated from T2416 (view on Phabricator)