Unreliable test for multi-zip POST

[sdumetz]

This test has failed at least once in CI :

 1) POST /scenes
       as use
         Fails on importing a zip with one or more scene with errors:
     Error: [404] bar/articles/hello.html
      at CleanVfs.getFileProps (file:///home/runner/work/eCorpus/eCorpus/source/server/vfs/Files.ts:197:19)
      at processTicksAndRejections (node:internal/process/task_queues:95:5)
      at Context.<anonymous> (file:///home/runner/work/eCorpus/eCorpus/source/server/routes/scenes/post.test.ts:330:20)