Possible flaky test in backup-restore test suite

Context: Build failure at https://gitlab.com/charts/gitlab/-/jobs/106221269

Test that checks if images uploads to issue is loaded correctly is possibly flaky. This is because, we are testing whether the address defined by src attribute of the image is returning 200. But, on checking the source code of the issue, we see this (split to multiple lines for readability)

<div class="wiki">
  <p dir="auto">
    <a class="no-attachment-icon" href="/root/testproject1/uploads/90701344e9ebb53fa9ebac83d43afdcc/Screen_Shot_2018-05-01_at_2.53.34_PM.png" target="_blank" rel="noopener noreferrer">
        <img src="data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==" alt="Screen_Shot_2018-05-01_at_2.53.34_PM" class="lazy" data-src="/root/testproject1/uploads/90701344e9ebb53fa9ebac83d43afdcc/Screen_Shot_2018-05-01_at_2.53.34_PM.png">
    </a>
  </p>
</div>

The image has lazy class, which means it is being lazy loaded. So, src is not pointing to the actual image, but data-src is. Once the image is loaded, src gets replaced by data-src. So, if the test checks for src, before the lazy loading is done, it gets the wrong data.

Proposal

Check if file mentioned in data-src (instead of src) is available and returns 200.

/cc @WarheadsSE